The Daily Mirror has sold more than a million extra copies since the start of its serialisation of Paul Burrell?s book, coming close to recouping the record £500,000 it paid for the newspaper rights.

According to industry sources the paper has put on about 200,000 sales a day since dropping its first bombshell last Monday, when it revealed Diana, Princess of Wales feared she would be killed in a car crash.

Insiders say this, combined with the huge amount of publicity the paper has enjoyed, has almost cancelled out the record payment for the rights to serialise Burrell?s book, A Royal Duty.

Although it did not achieve the hoped-for 400,000 extra daily sales, the Mirror was buoyed by the huge attention it received, with rival tabloids devoting pages of newsprint to following up its revelations. Mr Burrell?s claims also regularly made the evening news bulletins on TV.

The £500,000 sum is one of the biggest ever serialisation deals and puts the former royal butler behind only David Beckham in this year?s serialisation league.

Beckham?s biography, which the News of the World paid more than £1m to serialise, was the only book to sell for a higher price this year.

Such high payments are rare and reflect the quality of the material Mr Burrell had to offer.

Before the Beckham deal Margaret Thatcher?s was the only autobiography to reach seven figures ? the Sunday Times is believed to have paid more than £1m for the Iron Lady?s memoirs as part of a £3m deal with Murdoch-owned publisher Harper Collins.

The Daily Mail paid £900,000 for the rights to Victoria Beckham?s autobiography Learning to Fly and about £700,000 for Ulrika Jonsson?s memoirs, Honest.

This was despite the paper?s failure to pick up on Jonsson?s biggest revelation, that the television presenter had been the victim of a date rape committed by a man whom she did not name.

That bombshell only came out when she appeared in a television documentary.

Mr Burrell?s £500,000 fee is understood to be the highest ever paid by the Mirror and reflects the number of stories the paper has been able to squeeze out of his book.

The revelation that Princess Diana believed there was a plot to kill her by tampering with the brakes in her car was followed up by an extraordinary letter from her brother, Charles Spencer, in which he accused her of being ?manipulative and deceitful? and expressed concern for her mental health.

Yesterday the paper published a letter from the princess to Mr Burrell in which she referred to a secret that would change both their lives, although the paper does not reveal what the secret was.

Witness of the sex act

Today Mr Burrell claims the secrets of the so-called ?rape tape?, containing allegations made by the former royal servant George Smith, are ?very personal, very unpleasant, very damaging?.

The tape, recorded by Princess Diana in 1996, includes Mr Smith?s explosive claim that he witnessed a sex act involving a high-ranking member of the royal family and a servant.

On it, Mr Smith also alleges he was raped by a royal aide ? a complaint that failed to get to court last year because of insufficient evidence.

Although Mr Burrell admits he has not heard the tape in full, he said the princess told him what was on it, adding that it contained ?intimate details about other people?s lives?.

To find so many major revelations in a celebrity memoir is rare ? many have just one killer fact to drive the reader through a dramatic front-page story.

The newspaper declined to comment on the circulation rises, which are understood to be among the biggest for a Mirror exclusive.

Only its last Burrell buy-up ? when the paper paid the former royal butler £400,000 to tell his story following the collapse of his trial ? gave it a bigger circulation boost, adding around 1.5 million sales.

Claire Cozens

?THE RAPE TAPE?

Italian paper threatens to publish Diana ?secret?

An Italian newspaper is reportedly planning to publish details of the royal ?secret? that Princess Diana recorded on tape and told her butler, Paul Burrell, before she died.

Mr Burrell has vowed to take this secret to his grave ? but speculation about the contents of the elusive tape has reached feverish proportions ever since he made his claims about an ?explosive? fact that could destroy the royal family.

The royals fear it is only a matter of time before details emerge abroad or on the internet. In November ?La Repubblica? revealed the identity of the man accused of raping a male servant in Prince Charles? household, information which came from the same tape.

In the tape, George Smith claims that he was raped by an aide of the prince and also that he witnessed a sex act involving a royal.

According to reports the Italian newspaper would have published details of the taped secret today but journalists in Italy were on strike.

Mr Burrell said on BBC TV last night that Princess Diana had told him the secrets of the royal ?rape tape? were ?personal, and very damaging. It?s not very pleasant,? he said.