THE lawyers of Sir Paul McCartney and the former model Heather Mills will be anxiously awaiting next week’s House of Lords divorce ruling after the couple announced their separation yesterday.

Depending on the judgment, the former Beatle could stand to lose up to a quarter of his £825 million fortune if they divorce, experts say.

The law lords’ ruling on Miller v Miller and McFarlane v McFarlane promises to alter the landscape of family law. The cases concern the rights of a “homemaker” wife over her “breadwinner” husband.

Sir Paul and Lady McCartney did not sign a pre-nuptial agreement before their wedding in 2002. Lawyers said that it would have had little force under British law anyway.

Last night Sir Paul loyally defended his estranged wife against rumours that she was a gold digger who had targeted him while he was still mourning the death in 1998 of his first wife, Linda. In a note posted on his website, Sir Paul, 63, called Lady McCartney, 38, a “generous person who spends most of her time trying to help others in greater need than herself”.

He added: “It’s been suggested that she married me for the money and there is not an ounce of truth in this.”

Earlier the couple said that they cared about each other “very much”, but had found it increasingly difficult to maintain a relationship in the public eye.

Their daughter, Beatrice, is now two years old.

Sir Paul and Lady McCartney met at an awards ceremony in 1999. He was the wealthiest musician in the world. She was a model who had put a traumatic childhood behind her to become a passionate activist.

She campaigned for animal rights, against landmines and for people who had lost limbs. Her own leg was amputated after a collision with a police motorcycle in 1993.

Last year Sir Paul defended his wife from allegations of a rift between her and his three children from his previous marriage: James, Mary and Stella.In March the couple flew to Canada to protest before the annual slaughter of harp seal pups. In a BBC documentary broadcast last month they spoke out against the fur trade.But rumours of a split were fuelled last week when Sir Paul travelled to the French village of Beaulieu-sur-Mer, near Nice, without his wife.

Last night it was clear that their separation was expected to be permanent. Lady McCartney stands to gain a huge slice of her husband’s fortune. The case of Alan Miller, a City fund manager, is particularly relevant. He is contesting a Court of Appeal ruling that awarded his wife Melissa a quarter of his £20 million fortune for a marriage that lasted less than three years.

The head of one leading family law practice said: “Heather is going to get a very large amount of money. She could make £200 million, based on what he has earned during the marriage, the cost of rearing their child and the standard of living she is now entitled to expect.”

Sandra Davis, head of family law at Mishcon de Reya, said: “The glass ceiling for women has been broken and we have become more like America, where judgments have long been substantially more favourable to women than used to be the case here.”

STATEMENT FROM SIR PAUL AND LADY McCARTNEY

“Having tried exceptionally hard to make our relationship work given the daily pressures surrounding us, it is with sadness that we have decided to go our separate ways.

“Our parting is amicable and both of us still care about each other very much but have found it increasingly difficult to maintain a normal relationship with constant intrusion into our private lives, and we have actively tried to protect the privacy of our child.

“Separation for any couple is difficult enough, but to have to go through this so publicly, especially with a small daughter, is immensely stressful.

“We hope, for the sake of our baby daughter, that we will be given some space and time to get through this difficult period.”