The US administration and key allies around the world are braced for the release of a vast quantity of sensitive diplomatic files on whistleblower website WikiLeaks.
In Washington, the State Department denounced the leak of classified material as "irresponsible" and warned that it would place lives at risk.
Downing Street said the US ambassador Louis Susman had briefed British officials about the likely contents of the files which, it is thought, will include papers relating to the UK.
But the Prime Minister's the spokesman said: "I don't want to speculate about precisely what is going to be leaked before it is leaked."
State Department spokesman PJ Crowley said US diplomats were continuing the process of warning governments around the world about what might be in the documents. We are all bracing for what may be coming and condemn WikiLeaks for the release of classified material," he said. "It will place lives and interests at risk. It is irresponsible."
Many fear the cables will embarrass the United States and its allies, and reveal sensitive details of how the US conducts relations with other countries. It is thought that the documents may include reports from officials in Washington and diplomatic posts around the world about issues on which the UK and US have collaborated closely, including the wars in Iraq and Afghanistan.
Washington could be embarrassed by the publication of candid and forthright assessments of foreign governments made by its officials.
WikiLeaks - which has previously posted secret US files relating to the conflicts in Iraq and Afghanistan on the web - has not been specific about the timing of any release, which is widely expected to happen this weekend.
Mr Crowley warned that publication could erode trust in the US as a diplomatic partner. "When this confidence is betrayed and ends up on the front pages of newspapers or lead stories on television or radio, it has an impact," he said.
The State Department "has known all along" that WikiLeaks possesses classified documents, but it was not possible to predict exactly what information would be made public and what impact it would make, he said. "We wish this would not happen, but we are obviously prepared for the possibility that it will," he said.