The British government has insisted it does wield influence in Washington and does not fully support US-backed proposals from Israel in response to an attack by more than 50 former diplomats.
In an unprecedented letter signed by 52 former British ambassadors, high commissioners and governors yesterday, British Prime Minister Tony Blair was urged to start steering America's "doomed" policy in the Middle East or stop backing it.
The letter said hopes that US President George W. Bush would advance a "road map" for peace between Israeli and Palestinians had been dashed, and it attacked his decision to endorse an Israeli plan to retain some settlements in the West Bank as an illegal and one-sided step.
"We are very closely engaged, certainly on the road map process, with the United States," foreign office minister Mr Mike O'Brien told BBC Radio, in a first ministerial response.
"We have had some influence in encouraging President Bush to become the first US president to call for a Palestinian state, independent of Israel."
But he could not say whether President Bush had consulted Mr Blair before backing Israeli Prime Minister Mr Ariel Sharon's proposal, which has caused outrage among Arab leaders.
Mr O'Brien, while careful to play down differences with Washington, said Britain rejected key parts of Mr Sharon's plan. "We not accept that settlements have a legal right to remain on the West Bank. We do not accept that," he said.
Mr Blair has long maintained that a resolution of the Middle East peace process - a key grievance for militant Islamic groups - is vital to winning the "war on terror".
But he publicly backed President Bush despite a backlash in the Arab world over his endorsement of Israel's right to hold on to some West Bank settlements on land taken in the 1967 Middle East war.
The former envoys also attacked Mr Blair's staunch support for the US-led war in Iraq and the struggle now to establish stability after weeks of renewed bloodshed.
Opposition politicians said Blair would be well advised to take the envoys' criticism on board.
"This is a most remarkable intervention in the debate about the Middle East from a group of people who are almost certainly the most expert in Britain on the issue," Liberal Democrat foreign affairs spokesman Mr Menzies Campbell said.
Mr O'Brien said the government did take it seriously. "This is a . . . cry of frustration that things are not going as quickly as we would all like," he said of the letter.