It will cost you anything from £2,500 to £30,000 and it is likely to put a damper on the passion surging through your loins. But, one way or another, the prenup is coming to Britain.

The prenuptial agreement, a pre-wedding contract that says who gets what in the event of a divorce, is growing in popularity. It is now, say City lawyers, “almost commonplace” and not just for the rich.

Divorce planning is rapidly becoming as essential as tax planning. Four out of 10 marriages crash and burn and the costs can be horrific.

Two years ago Stephen Marks, founder of French Connection, the fashion chain, sold £36.5m of shares to pay for the break-up of his marriage, making his former wife Alisa one of the richest women in Britain — along with Pamela Morgan Bell, who picked up £100m when she split from Steve Morgan, a construction tycoon. Everybody is waiting to see how much Sir Paul No-Prenup McCartney will have to pay for the ending of his four-year marriage to Heather Mills.

Is the prenup a panic measure, will it work and is it a good idea? Opinions are deeply divided.

The issue has flared up because last week the House of Lords ruled on two spectacular divorce cases — Miller vs Miller and McFarlane vs McFarlane. Thanks to the law lords, Melissa Miller has walked away with £5m from her husband Alan’s £30m plus fortune and Julia McFarlane is to get £250,000 a year for life from her husband’s gross annual income of almost £1m.

The 25,000-word ruling — by five law lords led by Lord Nicholls of Birkenhead — is one of the most important decisions about British divorce ever made. It attempts to sort out the unpredictability of the rulings of lower courts and has already become a towering landmark in our long and colourful divorce history.

The point about the Miller and McFarlane cases is that they cover almost all the outstanding problems in divorce law in England and Wales.

Melissa Miller, an American, gave up an £85,000 a year job and a home to settle down and start a family with Alan, a tough City type, who was worth millions and was helping to start New Star, a fund management company. There were no children after Melissa had a miscarriage.

They separated in April 2003 because Alan was seeing another woman. Melissa was awarded £5m partly because, said the Court of Appeal, of Alan’s poor conduct.

Nicholls crushed the poor conduct argument as being against the convention of “no fault” divorces; but he upheld the £5m award because of the explosion in value of the New Star shares — they had more than doubled Alan’s wealth — and because of Melissa’s sacrifice of her own job and in providing domestic support for Alan. She was to be compensated for her part in his success and the damage done to her expectations and career.

The McFarlane case was utterly different, involving children and high income but low assets. The couple lived together for two years before they married in 1984. Julia gave up her job as a solicitor to look after their children — they were to have three.

When they separated in December 2000, they agreed to split their assets: a £1.5m house in Barnes, south London, a £255,000 holiday home in Devon and a £415,000 flat in Clerkenwell, north London, bought by Kenneth in June 2000. But they could not agree on maintenance.

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