The letter - an unusually strident public critique signed by 26 former military and foreign-service officials - says Bush's policies have proved ineffective and left the United States isolated internationally, according to the Los Angeles Times and the Washington Post on Sunday.
The signatories include officials appointed by presidents of both parties, who have served on every continent, including nations like Israel, the Soviet Union and Saudi Arabia.
"We just felt things were so serious, that America's leadership role in the world has been attenuated to such a terrible degree by both the style and the substance of the administration's approach," William C Harrop, ambassador to Israel under Bush's father, told the LA Times.
"A lot of people felt the work they had done over their lifetime in trying to build a situation in which the United States was respected and could lead the rest of the world was now undermined by this administration - by the arrogance, by the refusal to listen to others, the scorn for multilateral organisations," Harrop said.
"It is clear that the statement calls for the defeat of the administration," Harrop said.
"We just felt things
were so serious, that America's leadership
role in the world has been attenuated to
such a terrible degree
by both the style and
the substance of
the administration's approach"
William C Harrop,
former US ambassador to Israel
The group, calling itself Diplomats and Military Commanders for Change, plans to release the letter on Wednesday in Washington.
The public criticism comes amid rising public scepticism about Bush's handling of the war in Iraq, and less than five months before the presidential election.
Bush campaign officials declined to comment on the letter before its release.
One top strategist from Bush's Republican party said he did not think the letter would cause much political fallout.
"Their timing is a little off, particularly in the aftermath of the most recent UN resolution," the strategist told the paper, referring to last week's unanimous Security Council vote endorsing the handover of power in Iraq.
Bush also came under attack last month from former diplomats, who faulted his endorsement of Israeli Prime Minister Ariel Sharon's controversial plan to withdraw from the Gaza Strip.
That group of 58 former US ambassadors and diplomats said Bush had cost the United States "credibility, prestige and friends" and "placed US diplomats, civilians and military doing their jobs overseas in an untenable and even dangerous position".