Gordon Brown suffered a devastating new blow to his authority last night with the publication of a secret memo revealing Tony Blair’s true opinion of his performance.

The former prime minister made a scathing attack on his successor in a memo to a Labour colleague last autumn.

The secret document contains a catalogue of criticisms of Brown’s performance since taking office, accusing him of playing into David Cameron’s hands with a “lamentable confusion of tactics and strategy”.

Blair warns that the embattled prime minister may have made a “fatal mistake” by “dissing” the government’s own record to cash in on “anti-TB [Tony Blair] feeling” when he had “nothing to put in its place”.

The extraordinary memo, written in the aftermath of Brown’s disastrous decision to abandon plans for a snap election, threatens to undermine further the prime minister’s position after calls from Labour MPs for a leadership challenge.

The prime minister is facing the threat of an autumn coup,with David Miliband, the foreign secretary, and Alan Johnson, the health secretary, regarded as his most dangerous rivals.

Last night the former cabinet minister Stephen Byers fuelled Brown’s woes, accusing him of overseeing policies more fit for a “Sunday afternoon stroll” than winning the next election. His words signal the end of the self-imposed silence of Blairite ex-ministers privately critical of the prime minister — an alarming development for No 10.

Last night Downing Street launched a desperate damage-limitation operation. The Labour MP Khalid Mahmood, a loyalist, said: “Tony Blair should tell his friends to stop causing trouble and let Gordon get on with the job.”

Publicly, Blair continues to protest his unflinching support for the former chancellor. However, his memo suggests he has little faith in Brown. He warns “there is every indication that the lessons will not be learnt”.

Brown is to press ahead with his marathon visit to the Beijing Olympics despite warnings that Labour plotters will exploit his absence. His determination to spend five days in China has led to warnings of a repeat of “Thatcher in Paris” — the 1990 revolt against the Conservative prime minister that took place when she was in France.

A defiant Downing Street aide refused to accept his leadership was in jeopardy, challenging critics of the prime minister to do their worst. “Let them get on with it,” he said. David Miliband, the foreign secretary, whose newspaper article sparked the latest leadership speculation, faced a backlash last night from Brown loyalists — led by his brother Ed, the Cabinet Office minister.

A close friend of Ed Miliband indicated that he had been irritated by David’s bid to succeed Brown as prime minister. “Ed’s loyalty to Gordon is beyond doubt,” said the friend. “He is not his brother’s keeper. He is his own man. Ed is not to be held accountable for his brother’s actions.”

Writing in The Sunday Times, John Denham, the universities secretary, delivered a coded attack on David Miliband. “The question isn’t whether a few policies or a new face will get the voters listening but whether we really believe Labour is still the party that Britain needs,” he said.

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