President Bush responded to reports of a mass phone-monitoring campaign since the 11 September atrocities with a press conference at which he denied that his administration had done anything illegal.

The row broke out after a report in USA Today citing anonymous sources which alleged that major telecoms companies, including AT& T, Verizon and BellSouth, had been handing over all customer call records to the US National Security Agency (NSA) since 11 September. 

The NSA handles US electronic security. Denver-based Qwest Communications was the only firm to refuse to comply. 

Calls were not recorded but the caller and recipient of all domestic and international numbers has been and is being analysed in what is thought to be the world's largest database of its kind.

One person who, like the others who agreed to talk about the NSA's activities, declined to be identified by name or affiliation told the paper: " It's the largest database ever assembled in the world."

The NSA's goal, according to the source, is to "create a database of every call ever made" within the nation's borders.

President Bush said at the press conference: "The intelligence activities I authorised are lawful and have been briefed to appropriate members of Congress, both Republican and Democrat.

"The privacy of ordinary Americans is fiercely protected in all our activities. We are not mining or trolling through the personal lives of millions of innocent Americans. Our efforts are focused on links to al-Qaeda and their known affiliates."

But the allegations have drawn an angry response from politicians from all sides.

Republican senator Arlen Specter, chairman of the Senate Judiciary Committee, said that he would be formally requesting representatives from the telecoms companies to explain their actions, and would subpoena them if necessary.

Democrat senator Patrick Leahy asked: "Are you telling me that tens of millions of Americans are involved with al-Qaeda? These are tens of millions of Americans who are not suspected of anything."

The row has placed acute pressure on the White House as US Air Force General Michael Hayden, who oversaw the programme as the then head of the NSA, has been nominated by the White House to be the next head of the CIA.