Heather Mills is set to sue her former divorce lawyers for bad advice - even though she already owes them £2million in fees.

The latest legal action comes as Ms Mills prepares to go face-to-face with her estranged husband Sir Paul McCartney for a dramatic five-day divorce hearing that will decide how much she receives from his estimated £825 million fortune.

Ms Mills, 40, will represent herself at the High Court hearing, which begins on Monday, having split from her divorce lawyers Mishcon de Reya. Insolvency lawyers at Mishcons are now pursuing Ms Mills for almost 20 months of unpaid bills and are expected to launch a legal action in the next two weeks.

A source close to Ms Mills told the Evening Standard: "There is a possibility of Heather Mills counter-suing. Whatever she owes Mishcons - whether it's £1 million or £2 million - one would have expected her to get something for it and one fails to see what they [Mishcons] got her. There has been no divorce settlement and no outcome over all this time so draw your own conclusion as to the service she received from Mishcons."

The accusations baffled lawyers at Mishcons, one of Britain's top divorce firms, which found itself frustrated in trying to reach a settlement in a divorce case which grew increasingly acrimonious.

Sources said the firm will defend the advice it gave Ms Mills and suggested lawyers were frustrated that she often refused to take the legal advice given.

The slur will raise eyebrows as Ms Mills was being represented by Anthony Julius, the Mishcon litigator who also handled Princess Diana's divorce from Prince Charles and whose wife died from cancer in October.

Mishcons has now put the matter into the hands of its head of insolvency

But a source close to Ms Mills said she would resist any claim and counter sue, embroiling the former topless model, who lost a leg when hit by a police motorcycle, in yet more legal action. Danny Davies, who specialises in the "maximisation of business recovery". He is an expert in all aspects of insolvency law, including asset recovery and advises in bankruptcy cases, according to the law firm's website.

A source at Mishcons said today: "We expect a claim to go in against Ms Mills in the next few weeks."

The Standard understands that - as McCartney and Ms Mills head for next week's court hearing - the two sides remain "miles apart" with Sir Paul, 65, offering Ms Mills in the region of £15 million for their fouryear marriage while Ms Mills is holding out for around £50 million. The couple are also in dispute over the future ownership of a number of properties in London and elsewhere in Britain and abroad and over a confidentiality clause. Ms Mills is reluctant to be gagged by the former Beatle who is equally keen to prevent his estranged wife airing their dirty linen in public.

Ms Mills claimed in a divorce petition, subsequently leaked to newspapers, that Sir Paulwas violent towards her on four occasions during their marriage, including stabbing her in the arm with a broken wine glass.

He has vehemently denied the claims, prompting Ms Mills to start legal action against the Evening Standard for his dismissal of the allegations as "outrageous lies". Mishcons was also handling the libel action on a no win-no fee basis but may now seek to recover costs for that work too.

In television interviews in October, Ms Mills caused alarm with a series of rants in which she claimed - among other things: "I've had worse press than a paedophile or a murderer and I've done nothing but charity for 20 years."

The interviews led to her being dumped by her publicist Phil Hall and subsequently parting from Mishcons, which had also been advising her not to speak out until the divorce case was settled.