To back his case, he released a copy of a letter to Tony Blair providing evidence of the ill feeling that existed between No 10 and the corporation even before the BBC's controversial report claiming the government's dossier on Iraq's weapons of mass destruction (WMD) was "sexed up".

Dyke's intervention threatens to wreck the truce patched together last week when Lord Ryder, acting chairman of the BBC, made an "unreserved" apology to the government after the Hutton inquiry heavily criticised the corporation and cleared the government.

The extent of public scepticism about the Hutton report is shown in a YouGov poll for today's Jonathan Dimbleby programme on ITV. More than half (55%) believe it was a whitewash, while 26% think it was balanced.

Despite Hutton, 54% believe that the government "sexed up" the dossier on Iraq's weapons — one of the central allegations made by BBC reporter Andrew Gilligan in his Today programme report.

Hutton also faces a possible legal challenge on an important point of law on the rights of the media, established by a judgment involving The Sunday Times. Anthony Scrivener QC, a former chairman of the Bar Council and an expert on appeals, said Hutton had misunderstood the principle of freedom of expression, enshrined in the Human Rights Act.

It has also emerged that No 10 would like a Tory grandee, such as John Major or Chris Patten, as the next BBC chairman to counter criticism that it is intent on crushing the corporation. Blair has told Tessa Jowell, the culture secretary, to move swiftly to find a replacement.

Dyke said he had decided to breach an assurance of confidentiality to Blair because the letter, written on March 21, 2003, "demonstrates the intense pressure No 10 was putting on the BBC" during the war. It was written in reply to a critical letter from Blair, which questioned the BBC's journalistic output on the war.

The only clue to the letter's existence is on the Hutton website where Alastair Campbell, then the prime minister's director of communications, angrily describes it as "dismissive" of Blair's concerns.

In response to inquiries by The Sunday Times, Dyke said this weekend that Hutton "completely failed to acknowledge the pressure that No 10 had been putting on us". He said that Campbell had been waging a "war of attrition" with the corporation "at a time when the BBC was trying to report a very difficult story fairly and properly".

He revealed that Campbell had demanded the withdrawal from Baghdad of BBC reporters such as Rageh Omaar, claiming they were "compromised".

He said Campbell had sent letters to Richard Sambrook, the BBC's director of news, attacking the BBC's coverage of Iraq "week in and week out for a period last year". "It was a classic case of the Downing Street press office trying to intimidate the BBC," he added.

Dyke's further disclosure that he still has a private letter from Blair attacking the BBC is likely to alarm No 10. Dyke said this weekend that it would not be appropriate to release it.

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