I’m thrilled my husband will get on with my male greatest buddy… It means he’ll by no means guess we have been having an affair for THIRTY years

Walking through Palma airport, it’s fair to say I can’t get through passport control quick enough. Waiting for me in the arrivals hall is my best friend of 30 years, who I met at university.

It’s been a year since we last met and to say we’ve missed each other would be an understatement.

I see the homemade sign first, ‘Welcome Laura!’ and then behind it my friend’s beaming smile.

Three hours earlier my husband Jonny had dropped me off at the airport, kissing me goodbye and telling me to have fun. He knows how precious my old friend is to me and how much I’m looking forward to our weekend together.

And I really am, because, for the next 48 hours, sleep is off the cards – and passionate love-making is very much on them.

As I approach, my best friend drops the welcome sign and pulls me into a tender embrace. Before I know it, we’re kissing heatedly.

My best friend isn’t a woman. It’s Tim, a man I have known for longer than my husband.

Godfather to each of my three sons, Tim is an integral part of my life: the man I can tell anything to, who understands me on a cellular level – and with whom I have the most almighty sexual spark.

Married or not, there is no way I could ever countenance not being friends with Tim – or living without our electric love-making sessions, which have continued almost as long as my marriage.

You may wonder why I didn’t just marry Tim. That’s because, as sexually compatible as we are, outside the bedroom we’re too different.

I live within ten minutes of where I grew up in Wiltshire and adore my quiet country life, whereas Tim was a digital nomad before the term was even invented. While he’s currently living on the Spanish island of Mallorca for work, I have no doubt the next time we meet up he’ll have moved again to another exotic location.

For all the fireworks between us, however, there’s more to our relationship than just sex. We were best friends long before we slept together. And while you may snort with disbelief to read this, today I firmly believe the fact we have sex is secondary to our friendship.

That said, there’s no way I could ever stop having sex with Tim.

In many ways, I’m the last woman you’d expect to have an affair.

Physically, I’m very much Mrs Average. Yes, my fiftysomething figure is slim, but nothing to draw attention, and I’ve had the same chin-length brown bob since my teenage days.

Aside from Tim, I’ve never once looked at another man. Jonny and me have our own business – a TV production company based in the village where we’ve lived since we wed. We’re very much seen as a rock-solid couple, with three children, two labradors and a cat.

I admit, my public image amuses me, especially when I think of how wild my weekends with Tim can be.

If the shoe was on the other foot there’s no way I’d tolerate my husband having such an intense friendship with another woman, never mind sleeping with her.

I know this makes me a hypocrite – something that gnaws at my conscience. But it also enrages me that Jonny is content for me to be so close to Tim. Clearly, he doesn’t see me as someone who could be sexually attractive to anyone but him.

I suppose on some deeper level, another reason I find my sexual escapades with Tim so intoxicatingly addictive is because they give me a secret sense of power, a thrill in my otherwise domestic life.

Sometimes I look around the living room, when my three sons and husband are stretched on the sofas around me, and wonder how they can’t sense the fiery passions that boil within me.

My own childhood wasn’t conventional. Mum had me aged 16, after an ill-advised fling with an older man. My grandparents raised me while mum was in and out of hospital with what today would probably be diagnosed as bipolar.

Thankfully, my grandparents were loving – but goodness they were strict. Boyfriends were forbidden; they weren’t going to risk their granddaughter making the same mistake as their daughter.

When I went to university to study English, it was to nearby Bristol. Living in halls was out of the question for the first year; I had to commute. Only in the second year did they allow me to get a room in student accommodation.

There I met Tim. He hasn’t changed since: tall, slim with blond hair and an easy charisma. Little wonder women fall at his feet.

For much of our first year we were just on hello-goodbye terms. His social life was wild, mine wasn’t.

Then, aged 19, I got a call from my grandfather telling me Mum had walked into a local lake, after deciding to take her own life.

I remember sliding down the wall, devastated. Tim found me and sat with me that night in my room as I wept. He then brought me home and, after the funeral, would sit with me most evenings.

It became a pattern for the next few months. We’d listen to music, he’d cook. Slowly I started to heal.

By the end of my second year, Tim was my best friend – and our relationship was entirely platonic. In the midst of such awful grief, romance was the last thing on my mind.

For our final year it made sense for us to move in together. Even my grandmother was OK about it: even though, as she put it, Tim was a ‘strapping lad’, she could also see how different we were. The chance of anything sexual happening seemed remote.

Tim would have girls back, but I never felt jealous. Now, though, I can see that was because when they left, there’d be just the two of us. For my part, I kept away from men, believing they brought trouble.

Then, in our last week together, everything changed. During one of our dinners – garlic pasta and two bottles of chianti – Tim confessed how seriously worried he’d been about me in the months after Mum’s death.

He played an album by The Cure I had come to think of as ‘ours’. We started to tipsily dance. Then we started kissing.

I woke up the next morning with Tim in bed beside to me. It had been my first time. He pulled me into his arms, telling me this ‘wouldn’t change anything between us, we’ll always be best friends’. He was right. We are still best friends.

We spent the rest of that final week in my bedroom or his. When we graduated, Tim flew to Bangkok for a planned gap year.

Despite neither of us making any promises, I mourned for Tim at first. All I had were monthly postcards. It was pretty obvious he was living the single life.

So, when Jonny walked into my life a year after Tim left, I was only too happy to let him in.

I was then a production secretary for a documentary company and Jonny, three years older than me, was a camera assistant. He had a cuddly rugby player build, all shoulders and chest, with dark curly hair and chocolate brown eyes. At our weekly work drinks, he asked me out.

Like me, Jonny was happiest going for country walks followed by a pub lunch. Sex between us was gentle and slow. Jonny was careful and considerate. I felt safe with him and we fell in love.

Two years later, he proposed. How could I say no?

Tim, meanwhile, was working in Australia, but promised he’d come home for my wedding. I genuinely couldn’t wait to introduce the two men in my life.

When they met the week before our wedding, I immediately noticed Tim was more toned than last time I’d hugged him.

As for Jonny, he was so happy to meet my ‘best buddy’. The affection between them was genuine, which made me happy.

Tim told me he thoroughly approved of my future husband, which didn’t surprise me because everyone loves Jonny.

I gave birth a year after marrying. I struggled with motherhood and dreaded following in my unstable mum’s footsteps.

On the fifth anniversary of her death, two years after we married, Jonny was away filming. I called Tim, who was on one of his infrequent trips back to the UK. He knew immediately why I was ringing – over the years on her anniversary, I’d usually hear from him.

He turned up on my doorstep as a surprise that night – and I knew I needed him. I’d told Jonny he was popping by. Jonny always welcomed Tim into our life whenever he was back in England.

On this occasion, though, one hug led to another and we ended up having sex in my kitchen. I’m not sure who instigated it, but I’m sure we both wanted it very much.

For all it signified in terms of my marriage, I didn’t feel a scintilla of regret. Tim, however, looked guilty. I told him not to be.

In a later email I made it clear I wouldn’t be telling Jonny what had happened and would be very much staying married – but I needed his presence in my life.

Tim, being Tim, replied: ‘I’m always here for you.’

The next time we saw one another was a year later at a mutual friend’s wedding. Tim had brought a new girlfriend: a tall, willowy yoga teacher.

During the weekend we still managed to have sex in my hotel room. The illicit nature of our intimacy excited me as much as the act itself. Deep down, I knew it shouldn’t be happening – even though I told myself that because it was ‘just Tim’ it didn’t count.

By the time I was 35, I had three children under ten and was focused on raising them, while Jonny forged ahead with a career as a television director. The boys knew Tim as ‘uncle’ and he faithfully returned for godfather duties.

Despite the endless domesticity, Tim and I make sure we meet up for a sizzling weekend every year or so and regularly keep in touch via text and emails.

The majority of our messages are normal, friendship stuff – but occasionally the conversations get steamy. And, yes, there are times I imagine I’m in Tim’s arms while Jonny and I make love.

It sounds like the perfect affair, but Jonny’s mum, I know, harbours suspicions. She and I aren’t close and she never refers to Tim by his name, instead calling him, ‘Laura’s friend’, which bothers me.

Our sons, now in their 20s, have never questioned our friendship. They’ve laughed over pictures of Uncle Tim and mum at university, but their curiosity ends there.

I dread to think what would happen if they found out, because Tim is like family to them. Indeed, my youngest recently spent a week with him in Spain.

For his part, Tim has never married or had children. After five years with yoga girl, he lived with a French woman in Morocco, before settling down with a South American woman, acting as stepdad to her son for a decade.

Jonny has met all of Tim’s girlfriends with me, and is always fascinated to hear about their lives. But after each visit from Tim he will drape his arm around my shoulders and whisper that he wouldn’t trade our life for Tim’s for anything.

Jonny and I recently celebrated our 30th wedding anniversary. I’m proud we’ve got this far.

I know what I am doing is wrong. Yet any time I look in the mirror and see an unfaithful wife, I’m good at justifying why decades of infidelity are acceptable.

I tell myself Tim is not only my best friend, but has always been my comfort blanket. That, I insist, is why it’s not cheating.

As I age, I feel ever more astounded that gorgeous Tim still wants ordinary old me, especially when his partners are always so stunning. I find it increasingly astonishing that there’s still a sexual pull between us.

But it’s there, trust me.

Hand on heart, Jonny has never been suspicious of us. But while I know he would be devastated to find out about my duplicity, I’m also sure about something else. If I was given an ultimatum, I have no intention of saying goodbye to my best friend and lover.

• Names have been changed