So a new survey has come up with the surprise revelation that women are just as likely to be unfaithful as men.

Why didn't anyone realise this earlier?

Simply, it seems, because we women are more wily. While men are likely to strut around bragging about their conquests - indeed, even exaggerating the scale of their extramarital infidelities - women, conversely, are apt to keep mum about theirs.

Two thousand women were questioned in a recent sex and relationship survey. One in six of them admitted to having adulterous sex.

But here's the revelation: while females almost invariably discover the truth when their spouses stray, women are adept at keeping their own infidelities secret.

Lovers: John Major and Edwina Currie kept their affair secret from the prying eyes of the press and family

The august actress Dame Eileen Atkins, in an interview this week, put it bluntly: 'Women are more successful at infidelity than men - they're better liars.'

None of this, of course, comes as a surprise to me. After all, I successfully hid an affair of my own for almost 20 years.

Today, it is common knowledge - I'll explain later why I decided to break my long vow of silence - that, during my Parliamentary career, I enjoyed a passionate four-year liaison with John Major, who later went on to become Conservative Prime Minister.

John was then a whip in Margaret Thatcher's government - a man perceived publicly as dependable, if not rather staid - and apparently happily married to his wife Norma.

I was a forthright back-bench MP, a mum of two young daughters and wife to my first husband, Ray.

Yet between the years of 1984 and 1988, John and I would meet regularly for fulfilling, fiery - yet utterly discreet - sex.

No one knew of the liaison. We evaded the sharp-eyed attention of the Westminster village gossips.

We failed to arouse the suspicion of an ever-watchful Press. Even our closest friends and families suspected nothing. And our spouses were completely oblivious.

How did I keep our secret? I could concur with Dame Eileen and admit that I'm an accomplished liar.

Actually, I prefer to use a kinder euphemism: I'm good at dissembling.

I also became adept at economising with the truth; clever at coy evasions and quick to exploit every available opportunity to mask my infidelity behind the facade of legitimate business.

I'll give an example: when the then Secretary of State called a - generally unpopular  -  early morning meeting on Monday, I'd secretly rejoice.

The early start gave me a justifiable reason for scurrying down from our constituency home in Derbyshire to my London flat to meet John on Sunday night.

I'd breezily inform Ray that I needed to 'complete some paperwork', and the excuse was sufficient to appease his suspicions. So, by cleverly combining such deceits with plausible halftruths, I covered my tracks.

Of course, I avoided all the obvious pitfalls. Anyone who sets themselves up in a 'love nest', for example, runs the risk of being rumbled - a change of habits is always a give-away.

Certainly, if I'd taken out a short-term lease on a new property, suspicions would have been aroused.

Instead, we just met at my place. And we chose our times carefully. As whip, John always knew if we would be staying late in the House to vote.

On the occasions when we were not detained there, we took advantage of the early end to the day's business and went off for an evening of passion together.

Our respective families, meanwhile, assumed we were detained on business.

Both John and I were equally keen to keep our affair secret. We both had a lot to lose.

Neither of us wanted to jeopardise our marriages; nor did we want to imperil our careers. So we neither wrote letters nor left careless, incriminating messages.

Any gifts we exchanged were judicious. I once gave John a paper knife. It looked like the sort of perfectly innocuous gift one colleague would give to another, but for both of us it was fraught with special significance because I knew that John would think of me every time he used it to open his morning mail.

To give him his due, John was careful and did well to keep our affair a secret. But the many meetings his job involved gave him plenty of excuses to be away from home.

I, on the other hand, had to work much harder to cover my tracks and became even more accomplished at devising ruses.

Why are women so good at hiding their affairs?

Philip Hodson of the British Association of Counselling and Psychotherapy believes prudence, kindness and a desire to 'protect' their families all play a part.

Women are also often constrained by economic necessity: preserving a judicious silence is a prerequisite of their financial survival - and they are programmed to keep the family unit together.

It also seems that men are almost universally oblivious to the signs that accompany their wives' infidelities.

Women often lose weight when they begin an affair. They buy new clothes, change their hairstyles and are often suffused with that give-away glow that is imparted by illicit sex. I know I emitted such a glow during my affair with John.

Secrecy and the frisson of fear that accompanies infidelity enhance the excitement of adultery. Perhaps I should feel guilty, but in truth, I don't regret a moment.

I look back with nostalgia on those heady years of my life: I had a job I loved, and the thrill of those clandestine meetings with John was augmented by our shared passion for politics.

So it was that I hid the truth from my unsuspecting husband. Only pain and disruption would ensue, I argued, if I confessed everything.

Men, conversely  - and this latest survey corroborates the fact - are prone to exaggerate their infidelities, even laying claim to affairs they've never had. Such bragging, they obviously believe, adds to their machismo.

I'm not suggesting for one minute that the former Liberal leader Paddy Ashdown committed adultery in order to enhance his status in the polls - but, actually, when his affair with his secretary was revealed, his popularity increased.

So why, having gone to considerable - and many would say devious - lengths to hide my affair with John, did I decide, 14 years after it had ended, to confess openly to it? The answer is simple: I felt a gross hypocrisy on John's part that needed to be exposed.

In 1999 he published his memoirs. I was, I confess, hurt that I merited not even a passing mention in them.

Worse, however, in my view, was his sanctimonious espousal of the Back To Basics policy.

This guiding principle of Conservatism was his brainchild - and underpinning it was his belief in 'family values'.

The audacity of the man! My desire to preserve our secret was superseded by a far more urgent compulsion to reveal his duplicity. So that's why, when I published my diaries in 2002, I revealed our affair.

My decision, of course, had major repercussions.

My exhusband, Ray - we had divorced in 1997 - was hurt. My grown-up daughters were shocked and horrified.

John, who said he was 'ashamed' of the affair, has spoken not a word to me since.

But I don't actually regret either my infidelity - or my decision to write about it.

I am happily remarried now, to John Jones - I call him JJ - whom I met in 1999 when he was a guest on the radio programme I presented.

He's a retired detective; a man of great understanding, humour and courage. I love him, he loves me; he's a great lover - and, before you ask, no I won't be having another affair.