The letter, which was obtained by Reuters, expresses the signatories' support for 52 retired British diplomats who also sent a letter to Prime Minister Tony Blair last week.
"We former diplomats applaud our 52 British colleagues who recently sent a letter to Prime Minister Tony Blair criticising his Middle East policy and calling on Britain to exert more influence over the United States," the US letter begins.
Harshly criticising Bush for his support for Israeli Prime Minister Ariel Sharon, the letter said: "Your unabashed support of Sharon's extra-judicial assassinations, Israel's Berlin-Wall-like barrier, its harsh military measures in occupied territories and now your endorsement of Sharon's unilateral plans are costing our country its credibility, prestige and friends."
According to Andrew Killgore, who served as US ambassador to Qatar from 1977 to 1980 and was coordinating the effort, the letter has been signed by several former ambassadors, including James Akins, who was US ambassador to Saudi Arabia from 1973 to 1976; Robert Keeley who was assistant secretary of state for African affairs from 1978 to 1980 and later ambassador to Zimbabwe and Greece; and John Gunther Dean, ambassador to India from 1985 to 1988.
"We're not the good guys any more and our foreign relations have been and are being damaged"
former under secretary of state for economic affairs, USA
Killgore said the group intended to go public on Tuesday with a Washington news conference. He said so far there were around 50 signatories.
Other senior former diplomats said they were considering joining and were deeply disturbed by the recent direction of US policy, not only in regard to the Middle East but also on human rights generally.
"We're not the good guys any more and our foreign relations have been and are being damaged. We are viewed as hypocritical," said William Rogers, who was Under Secretary of State for Economic Affairs in the mid-1970s. Rogers said he had not decided whether to sign the letter.
The diplomats said they were deeply concerned by Bush's endorsement last month of Sharon's plan to withdraw unilaterally from Gaza.
The plan was rejected on Sunday in a referendum of members of Sharon's ruling Likud Party, but the Israeli leader said he still wanted to press forward, although he might change some of the details.
The letter said that by backing the plan, Bush had tossed away the rights of three million Palestinians.
They said Bush had placed US diplomats, civilians and military overseas in an untenable and even dangerous position by pursuing an unbalanced Middle East policy.