Detectives carrying out the multimillion-pound investigation into illegal newsgathering techniques at Rupert Murdoch's British newspaper group have been asked to investigate whether it attempted to blackmail politicians.

The alleged plot centres on News International's apparent efforts to warn off MPs on a parliamentary committee from disproving its discredited defence that phone hacking was the work of a single "rogue reporter".

According to the former senior News of the World journalist Neville Thurlbeck, News International ordered the Sunday paper's reporters to scour the private lives of MPs on the Commons Culture, Media and Sport Committee in 2009. At the time, Mr Murdoch's company was mounting what it now admits was a mistakenly "aggressive" response to allegations that the interception of voicemail messages was rife at its headquarters in Wapping, east London. On the advice of the parliamentary authorities, the Labour MP Tom Watson has now asked the Metropolitan Police to investigate the allegation.

According to Mr Thurlbeck, reporters were told by those in "deepcarpetland" to obtain evidence of affairs or gay relationships. The aim, he claimed, was to "to find as much embarrassing sleaze on as many members as possible in order to blackmail them into backing off from its highly forensic inquiry into phone hacking". In a letter - a copy of which has been obtained by The Independent - to the Deputy Assistant Commissioner leading the Met's inquiries into News International, Sue Akers, Mr Watson wrote: "If these allegations are found to be true, it suggests there was a conspiracy to blackmail."

Mr Watson, a member of the committee put under surveillance by News International, said: "I have evidence that I was put under covert surveillance in September 2009 by the private investigator Derek Webb, as well as Mazher Mahmood and an accomplice. Mr Thurlbeck may not be aware of this. I would, therefore, urge you to investigate Mr Thurlbeck's claims in order to establish whether any offence was committed."

The Met launched an inquiry into alleged computer hacking at The Times earlier this year in response to a letter from Mr Watson. Yesterday the Met said it was unable to comment on the MP's latest request. The Leveson Inquiry - where former Labour Prime Minister Tony Blair will appear today - has been investigating the power wielded behind the scenes by News International. While blackmail would be a significant deepening of the criminal inquiries enveloping NI, former News of the World reporters have spoken in the past about their use of "leverage" to secure the co-operation of people about whom the paper had embarrassing information.

Mr Thurlbeck, the former chief reporter, disclosed the alleged operation against MPs in comments to Mr Watson that the MP included in his co-authored book, Dial M for Murdoch. Mr Thurlbeck said that an "edict" had been passed to reporters to "find out every single thing you can about every single [committee] member: who was gay, who had affairs, anything we can use."

In the New Statesman this month, he added that the NOTW's journalists had been so concerned about the exercise that they did not carry it out, but went further than he had previously about its intent. He wrote: "At the height of the hacking scandal, News of the World reporters were dispatched round the clock... the objective was to find as much embarrassing sleaze on as many members as possible in order to blackmail them into backing off from its highly forensic inquiry into phone hacking."

In his letter to Ms Akers, dated 9 May, Mr Watson wrote: "I think it is important that I write to make you aware of new information published by Neville Thurlbeck.... The comments concern the blackmail of members of the Culture, Media and Sport Select Committee, and will be of interest to you given Mr Thurlbeck's previous arrests."

Mr Thurlbeck, arrested on suspicion of phone hacking, remains on police bail. News International made no comment on Mr Watson's letter.

News International 'Tried To Blackmail Select Committee'

meanwhile .....

Tony Blair will be before Lord Justice Leveson on Monday, which will hopefully allow us to ask all the important questions. Why did the former prime minister decide to become godfather to Rupert Murdoch's daughter Grace? And why was he mysteriously cut from pictures taken by Hello! magazine of the happy event, modestly located in the same place where Jesus Christ was himself baptised. This, at least, will provide welcome relief from the slow motion battering of Jeremy Hunt - and perhaps an opportunity to rediscover the larger narrative.

After all, it can be argued that the modern media-political matrix began the moment Blair accepted an invitation to fly out to address Murdoch's top executives at Hayman Island, which for all the talk of mating porcupines (they work slowly, lest you be in any doubt) was in fact followed by rapid mutual penetration. Murdoch's newspapers rallied behind Blair; Labour types soon began to land jobs in the press offices and public affairs departments of News Corporation and BSkyB. There was nothing as vulgar as a formal deal either - because that would be to misunderstand what both wanted.

Labour was after political support of the kind that had eluded it for a generation, while ministers may have had an eye on columns and book contracts too. Murdoch and his allies wanted high-class gossip, a healthy supply of scoops, and generally to be left alone owning the biggest newspapers and what was becoming the largest broadcaster. Nobody needed to write down the terms; it just evolved and was understood.

Labour may have blocked the Man Utd takeover, although buying the club would have been a monumental distraction, and chucked a bit of cash at the BBC, but hardly enough to stop Sky seizing more sports rights. Blair enjoyed the longest of political honeymoons, although perhaps it was the backing of the Murdoch press that helped give him the reassurance that he could meet his promise to President Bush over the need to join him in invading Iraq, the war that ultimately finished off his popularity.

So it is entirely unsurprising that a fresh generation of ambitious politicians - David Cameron and George Osborne - and Murdoch media executives - James Murdoch and Rebekah Brooks - picked up the old red playbook and aimed to turn it blue. Except, by then, News International and Sky had grown relative to their rivals - and five years of hubristic phone hacking had been allowed to fester largely unnoticed. Plus the company had grown more overtly aggressive under new management, switching sides in the middle of a Labour conference and crying foul if things didn't go their way.

What emerged then was a culture of private, unminuted, intimate socialising. It was a given that the top echelons of News International could fix up dinners or other meetings with the top Conservatives. James Murdoch could meet Cameron for breakfast and quietly promise him that the Sun would swing round to him three weeks before it actually did in September 2009. Or James Murdoch could ring up Hunt in November 2010 in an effort to get round the annoying Vince Cable, and sound so unhappy that the culture secretary feels impelled to tell the prime minister how furious he is. There were dinners too for Osborne and Michael Gove, and a flight to Santorini for Cameron thrown in.

Perhaps all this social intimacy is enjoyed as frequently by executives from BP, Barclays and Vodafone. But it is more likely that the mutual interests of the media and politicians give rise to an inevitable closeness. Which is why, in a world of so many opinions, it matters that there is a media more diverse than competition law allows. It is too easy for prime ministers to give the best seats to the most powerful media types - and become part of each other's families too.

The Blair Essentials: How A Mating Porcupine Became A Point Of Habit