The Government has been briefed by US authorities about the expected release of sensitive diplomatic files on whistleblower website WikiLeaks, Downing Street confirmed today.
Reports suggest that the files could include hundreds of cables relating to UK interests, and might include revelations about secret intelligence sources and practices.
Washington is braced for the release within days of thousands of documents on the website, which has previously published secret details of allied military operations in Iraq and Afghanistan.
"These revelations are harmful to the US and our interests," said State Department spokesman PJ Crowley last night. "They are going to create tension in relationships between our diplomats and our friends around the world."
Prime Minister David Cameron's spokesman this morning declined to discuss the nature of any confidential communications which may have been obtained by WikiLeaks.
The spokesman said: "Obviously, the Government has been briefed by US officials, by the US ambassador, as to the likely content of these leaks.
"I don't want to speculate about precisely what is going to be leaked before it is leaked."
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 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.