LONDON — Sir Paul McCartney’s estranged wife, Heather, gave notice last night that their divorce will be one of the bloodiest in British legal history by hiring the lawyer who represented Princess Diana, Anthony Julius.

Mr. McCartney has retained Fiona Shackleton, who acted for Prince Charles in the 1996 divorce that ended in a $32.4 million settlement for Diana.

This time, the stakes are much higher, with Mrs. McCartney demanding a substantial share of her husband’s $1.6 billion fortune.

The former Beatle, the wealthiest survivor of the band, fears a huge settlement after the four-year marriage.

He is understood to have offered Mrs. McCartney $57 million. This is $34 million less than Britain’s biggest contested divorce payout by an insurance magnate, John Charman, who was married to his wife, Beverley, for 29 years and wanted her to accept $38 million.

Friends of Mrs. McCartney, who has said she will revert to her maiden name of Heather Mills after the divorce, had suggested that at one stage, she would have accepted $19 million, rather than a possible $381 million.

A series of surprises about her colorful past, including a photograph of her when she was a glamour model, surfaced, followed by suggestions that access arrangements for their 2-year-old daughter, Beatrice, may become part of the case.

She became furious when her husband stole a march on her by filing a divorce petition. He also appointed a member of the queen’s counsel, Nicholas Mostyn, known as “Mr. Payout” because of his formidable reputation.

The divorce papers cite Mrs. McCartney’s alleged unreasonable behavior. She is accused of being rude and argumentative to his staff.

Her side says she is lodging a counter-claim that he is a mean person and that once they were married, became boring and selfish.

Mr. McCartney, 64, has frozen their joint bank accounts and changed the locks on at least one of their houses, a $11.4 million London mansion near Lord’s cricket ground in St.John’s Wood.

A spokesman for Mishcon De Reya, where Mr. Julius is a senior consultant, said last night: “We can confirm that we have been instructed by Lady McCartney to act in her divorce proceedings. Her team is being led by Anthony Julius, along with the family law partner, Nicola Fletcher, and assistant, Laura Tyler.”

The rancor has set in only recently. In May, Mr. McCartney said their parting was “amicable” and that “we still care about each other very much.” They were determined to stay friends, for the sake of their child. His side said he wanted a “quickie” divorce.

But anger emerged when reports claimed Mrs. McCartney was a “gold digger,” which Mr. McCartney said he did not believe. He denied that she married him for his money.

The two have been dogged by public quarrels, particularly in the run-up to their marriage, when an engagement ring was flung from a Florida hotel room and Mr. McCartney had to hire a private detective to recover it.

The increasing bitterness was illustrated two nights ago when police were called to the St. John’s Wood house after Mrs. McCartney found herself locked out with Beatrice.

Mr. McCartney’s camp inferred that the 38-year-old former model, who has been followed everywhere by photographers, was trying to stage-manage the circumstances to win sympathy. But Mrs. McCartney insisted that she was mortified to find the gate’s locks had been changed.

She was unable to get in for two hours, and police were called to deal with an “intruder.” Yesterday, accompanied by two bodyguards and two assistants, she emerged from the house and left in a car. Twenty minutes later, Mr. McCartney arrived to collect Beatrice.

His spokesman said: “They are both working in Beatrice’s best interests at all times.”

Mrs. McCartney has changed her public relations team, hiring a former editor of the News of the World, Phil Hall.