Gordon Brown calls them "known criminals" but, to many journalists, the blaggers who dig up confidential material for cash are indispensable allies in their ceaseless quest for private information.

In his book Flat Earth News, the Guardian reporter Nick Davies trawls the trawlers and identifies some of the most successful – and notorious – blaggers ever to have sold their services to Fleet Street.

Barry Beardall, the man who tricked a firm of London solicitors into handing over details of the sale of a Westminster flat that Brown had bought in 1992, is described as "a frontman for the Sunday Times … [who] used his accounting skills to set up phoney companies with phoney accounts and phoney histories and then conducted investigations which were at best highly unorthodox and at worst exercises in pure entrapment".

As well as going after the former prime minister, Beardall also tried to con information from Tony Blair's financial advisers in 1997 and, posing as a property developer, approached aides to the Labour mayoral candidate Frank Dobson in May 2000 in a bid to get them to give him a political concession in return for a £10,000 campaign donation.

"He is particularly good in business meetings, juggling offshore funds and bottom-line figures," writes Davies. "And yet he is not qualified as a chartered accountant and eventually ended up in prison for fraud."

Also prominent in the buying and selling of information was Jonathan Rees, who ran a security business in south London that hired serving Metropolitan police officers who wished to moonlight.

The police he employed allowed him to establish a very useful network of contacts and he became "a prolific source of leaked information" for the News of the World, the Daily Mirror, Sunday Mirror and the Sunday Times.

Scotland Yard eventually became so anxious about his activities that they bugged his office.

His sources included:

• Tom Kingston, a detective constable in the south-east regional crime squad who went to prison for selling drugs.

• Duncan Hanrahan, who left the force to set up his own security outfit and was jailed for corruption.

• Martin King, another former policeman who suffered a similar fate.

• Austin Warnes, a detective constable who ended up in jail for conspiracy to pervert the course of justice.

• An officer from the Royal Protection Squad who was on a £150-a-month retainer to give information about the royal family and other VIPs.

An internal Met police report found that Rees and his colleagues had "for a number of years been involved in the long-term penetration of police intelligence sources… Their thirst for knowledge is driven by profit to be accrued from the media".

Davies notes: "Rees was so useful that News International were put off him neither when he was arrested on suspicion of murdering his business partner (and released without charge), nor when his more recent business partner, former Scotland Yard detective sergeant Sid Fillery, was convicted of possessing child pornography, nor when he was arrested for plotting to plant cocaine on the estranged wife of a client who hoped it would help him get custody of his child (for which Rees was sentenced to seven years in prison). News International records show that, even after he emerged from this prison sentence, he was still routinely selling them information, including telephone records, and had his own account with them."

Other former police officers set up their own specialist operations. One detective, whom Davies refers to for legal reasons only as Z, was kicked off the force following allegations that he took cash from criminals in return for burying evidence. He soon reinvented himself as a "conduit for corruption", carrying cash bribes from journalists to serving police officers in exchange for information. Senior police officers believe entire inquiries have had to be abandoned because of the leaks sold through Z.

But the most successful of the blaggers used by the Sunday Times, according to Davies, was "a former actor from Somerset named John Ford, who used the skills of his former profession as well as a gift for mimicry to blag his way into confidential databases, particularly those of British Telecom, building societies, credit card companies and banks".

In his chapter on blagging – entitled The Dark Arts – Davies reflects on those who gather and sell private and confidential information.

"Many journalists will protest that there is nothing wrong with any of this," he writes. "They will say that their job is to obtain information; that the state has no business deciding what should and should not be published; that this is simply the free press at work. Most of all they will say they are working in the public interest. They may prefer not to acknowledge that buying their way into confidential databases involves no skill and no professional satisfaction and is really no better than a fisherman who can't be bothered with a rod and line and just chucks some explosives into the lake instead. Certainly, they would deny that they could, in truth, be doing most of this blagging and bribing themselves and that a big attraction of hiring private investigators … is that, for the most part, it is they and not the journalists who will end up in the dock."