BBC Correspondent Steven Sackur says it may be some time before the agents' evidence is made public

The guards gave evidence to a grand jury investigating claims that the president told Ms Lewinsky to lie under oath about an alleged affair between them, after the Clinton administration lost an appeal to the Supreme Court.

Mr Clinton's lawyers had argued that if the guards were made to testify, future presidents would be reluctant to have security officers present when they held confidential discussions in case they were later repeated.

Lewinsky: denies wrongdoing

However, the Chief Justice of the Supreme Court, Judge William Rehnquist, ruled that the bodyguards did not have legal immunity.

He said the Clinton administration had failed to show that "irreparable harm" would result if the officers testified about what they saw or heard while protecting Mr Clinton.

A fourth secret service agent, Larry Cockell, the former head of Mr Clinton's security detail, also turned up to give evidence to the grand jury on Friday. But there was not enough time for him to be questioned.

Perjury probe

The investigation into the allegations against the president is being led by Special Prosecutor Kenneth Starr.

Both Mr Clinton and Miss Lewinsky have denied any wrongdoing.

Starr: stepping up pressure

Mr Starr had subpoenaed the secret service agents in order to ask them if they had ever seen Mr Clinton with Miss Lewinsky or overheard any conversations relating to the former White House intern.

The Supreme Court ruling came just minutes before the deadline for several secret service employees to give evidence before the grand jury.

President Clinton: "These people risk their lives to protect me"

The administration had argued that testimony by presidential bodyguards would undermine confidentiality and endanger security.

The grand jury has been hearing evidence for nearly six months about the sex-and-perjury allegations.