But, your Honour, I did once iron his shirt for him

The Law Commission is proposing that a man's mistress should have full legal rights - as long as there was more than sex involved. Tricky, says Sally Ann Lasson, but at least it will spell the end for all those smug marrieds out there

Mistress. Mmm. What a marvellous word. Makes one think of glossy women in airport novels, driving sports cars and carrying lots of little tiny carrier bags. Men love the word mistress, it makes them feel all warm and sexy and self-important. It makes them jiggle their buttocks and feel that all is right with the world; that women are devoted types who wait discreetly in silky underwear for the phone to ring.

Actually, mistress is a toy word that describes a toy role. Unless you are a producer for Carlton TV, you won't have used it much recently. And any sexy connotations the word retains will soon be legislated out of existence because the Law Commission has said that mistresses should be given full legal rights. Bye bye mistress, hello co-habitee with property and pension rights. Hasn't got quite the same frisson, has it?

Legally, a mistress is defined as someone having a sexual relationship with a married man who pays for her maintenance. The Law Commission suggests that if mistresses can prove they have contributed more than a "sexual role" to the relationship, they can expect a form of legal recognition. This will prove to be a very entertaining law in the making. Oh, the deliciously prurient evidence that will be required, first to demonstrate a sexual relationship, then to demonstrate more than a sexual relationship. If I didn't know any better, I might assume that lawyers were just a seedy bunch who don't get out much and have never really managed to recover from the excitement of that semen-stained dress.

Monica Lewinsky was not President Clinton's mistress, she was his lover. A hatpin doesn't make a mistress. But his defence will become typical. Men only know three sentences when they have been caught with their fingers in the pie: I didn't do it; I hardly did it; I didn't enjoy it. They will go to great lengths to assert that they didn't do something much if there is money at stake. There was quite a fuss at the inception of the Child Support Agency - children's rights, women's rights, where will it end? Another book by David Thomas about the oppression of the wealthy, privately educated white male, perhaps?

Of course, a female co-habitee with property and pension rights sounds suspiciously like something we are already quite familiar with: wives. I've never met a mistress, but I do know women who have minds like whores. The most successful of them marry extremely rich men and demurely sign pre-nups that their own lawyers have already told them aren't worth the paper they're written on. Sorry to drag up a very old chestnut, but in the movie Pretty Woman, Julia Roberts was quite happy to be a whore, and had ambitions to be a wife. What she was insulted by beyond measure was Richard Gere's suggestion that she be his mistress: finite shopping options. So unromantic.

But if every kind of live-in partner has rights, then the great prize of marriage will become obsolete. And any law which undermines the moral and legal authority of the church can only be enormous progress for women. When they earn good money, women increasingly find the lure of marriage resistible. There has been a 20 per cent fall in the number of weddings in the last 10 years. Almost half of all marriages end in divorce. More people are opting to live alone than ever before. When I recently asked a woman going through a sticky patch in her relationship what she'd rather keep, the house or the man, she couldn't answer me.

Sir James Goldsmith's oft-quoted remark about marrying your mistress and creating a job vacancy wasn't quite accurate in his case - it should have read vacancies. There was more than one woman circling round his carcass. His wife, Lady Annabel, has never made a particularly convincing mistress, great bovine hulk that she is. But then neither does her contemporary, Camilla Parker Bowles, who, if it weren't for this new law, one might be forgiven for thinking was the only mistress left in the country. Mrs Parker Bowles is a relic; she has an inherited genetic predisposition for sleeping with Princes of Wales. She is a proper mistress in that her boyfriend subsidises her lifestyle, as does her ex-husband. Let's not forget she was also an adulteress, and her husband a notorious adulterer and cuckold. (The happiest cuckold in Britain, perhaps.)

Mistress. Adulterer. Cuckold. If the law is to take into account a varied complexity of modern relationships, perhaps it is time to consign these words to the bin. The legal profession is planning ahead because any new laws relating to partnerships will keep them in full employment and fill the penurious void that is left when nobody bothers to get married and therefore, more importantly, nobody requires a divorce lawyer. They're merely diversifying.

The mistresses who might have got more...

Under the suggested new guidelines, diarist Alan Clark might have been dragged through the courts after sleeping with the wife and two daughters of Judge James Harkess. Valerie Harkess and her daughters Josephine and Alison (otherwise known as the "coven") could have bought a claim to Clark's 12th-century retreat, Saltwood Castle. Instead, the MP, who is said to be worth pounds 30m, escaped with the threat of "a horse-whipping".

Monica Lewinsky emerged from her relationship with President Clinton clutching a blanket, a hat-pin, a book deal and a slightly soiled dress. However, under the proposed new legislation, if she had been able to prove that she contributed more than a "sexual relationship", she could have walked away with a share of Clinton's property and pension plan.

Prince Charles's mistress, Camilla Parker Bowles, has had to endure ridicule and vitriol from the press... and for what? A few bits of jewellery and the odd night out? She may never be Queen, but she could stand to gain a fraction of his fortune. The revenue from his Duchy of Cornwall estate is worth an estimated pounds 5.9m a year.

...and the ones who didn't do so badly

Former chorus girl Marla Maples started off a mistress, and then traded up to become the wife of American tycoon Donald Trump. Although he signed a $1.3m prenuptial agreement before their wedding in 1993, Ms Maples Trump, as she is now known, stands to make even more. They are set to fight it out in the courts in the autumn. Marla, who bore him a daughter, Tiffany, said recently: "He promised to look after us."

In the absence of legal protection, many modern mistresses have to kiss and tell. In 1994, Bienvenida Buck sold her story after a fling with Sir Peter Harding, the married Chief of Defence Staff. Represented by Max Clifford, she won an estimated pounds 100,000.

57-year-old Ann Rowe, from Kent, dreamed of sailing around the world with her married lover, Edward Prance, in his yacht, Edwardiann Castle (an amalgamation of their names). When their relationship ran aground, 68-year-old Mr Prance decided to sell the boat and keep the pounds 170,000 proceeds. In a landmark move last week, the High Court ruled in his mistress's favour, and Ms Rowe was awarded half the profits of the sale.