In an unprecedented letter published on Monday by 52 former ambassadors, high commissioners and governors - the top ranks of British diplomacy - Blair was urged to sway US policy in the region as "a matter of the highest urgency".
The diplomats, among them former ambassadors to Iraq and Israel, told Blair they had "watched with deepening concern the policies which you have followed on the Arab-Israel problem and Iraq, in close cooperation with the United States.
"We feel the time has come to make our anxieties public, in the hope that they will be addressed in parliament and will lead to a fundamental reassessment," said the letter, sent to Blair and made available to the media.
A spokesman would not be drawn into how Blair would respond to the attack, which the diplomats believe is unprecedented in scope and scale.
The signatories say Iraq's war
dead number 10,000 to 15,000
"The prime minister rejects the idea that there is in some sense a score card between British objectives and US objectives," the aide told reporters.
"Britain's objective is a democracy in Iraq, a two-state solution in the Middle East; those are objectives that Britain will work with our allies, including the US, to achieve."
The protest letter comes as Blair faces deep discontent among voters for backing a US-led war that most Britons had opposed and for endorsing a Washington-driven policy that has put London on a collision course with allies in Europe.
The diplomatic swipe is bound to be seized upon by Blair critics as fresh evidence that British interests come second to America's because of Blair's zealous alliance with President George Bush and his neo-conservative agenda.
The diplomats zeroed in on two key initiatives dominated by Washington - Israeli-Palestinian peace negotiations and the war in Iraq - and wrote off both as "doomed to failure".
"Never has government policy been so controversial. It is an indication of our serious concern that what is probably the biggest ever such collective group has gone straight to government in this way," the letter's coordinator Oliver Miles, a former ambassador to Greece, told Reuters.
"Our objective is not to damage Blair politically but to strengthen the hand of those who feel as we do," said Miles. "Our voice will be heard."
"We feel the time has come to make our anxieties public, in
the hope that they will be addressed in parliament and will lead to a fundamental reassessment"
Among the signatories are former ambassadors to Baghdad and Tel Aviv, top Arabists and non-regional specialists who served from Moscow to Brussels to the United Nations.
The career diplomats urged Blair to use his alliance with Bush to exert "real influence as a loyal ally... If that is unacceptable or unwelcome, there is no case for supporting policies which are doomed to failure."
The diplomats criticised the toll of the war and apparent lack of a plan for life in the country post-Saddam.
"The Iraqis killed by coalition forces probably total between 10,000 and 15,000," they said, estimating the number killed in the last month in Falluja alone at several hundred.
Blair is also attacked for
endorsing Sharon's Gaza plan
"There was no effective plan for the post-Saddam settlement...To describe the resistance as led by terrorists,
fanatics and foreigners is neither convincing nor helpful."
On the Palestinian question, the diplomats said big powers had waited for US leadership to advance a "road map" for peace that had raised expectations of a lasting Israeli-Palestinian settlement.
Waiting in vain
"The hopes were ill-founded. Nothing effective has been done
either to move the negotiations forward or to curb the violence. Britain and the other sponsors of the road map merely waited on American leadership, but waited in vain," it said.
"Worse was to come," they continued, attacking Bush's decision this month to endorse an Israeli plan to retain some settlements in the West Bank as an illegal and one-sided step.
"Our dismay at this backward step is heightened by the fact that you yourself seem to have endorsed it, abandoning the principles which for nearly four decades have guided international efforts to restore peace in the Holy Land."
Blair has backed Bush in public but, privately, government sources fear the so-called road map is in tatters. The prime minister has also attacked Israel's assassination of two Hamas leaders, in sharp contrast to Washington.