Mr Clinton's lawyers had argued that the two men, Bruce Lindsey and Sidney Blumenthal, were covered by executive privilege and so should not have to testify in the case involving Monica Lewinsky.

However, the judge ruled that the importance of their evidence to the investigation meant they would have to do so.

Kenneth Starr: ordered voice, fingerprint and handwriting samples

Meanwhile, the special prosecutor Kenneth Starr has ordered Monica Lewinsky to provide voice and handwriting samples and fingerprints.

President Clinton and Monica Lewinsky have both denied under oath that they ever had a sexual relationship.

The special prosecutor's team has spent months trying to prove that they had, and that they lied to lawyers who were investigating charges of sexual harassment against the president.

Mr Starr's request for voice, handwriting and fingerprint samples could be in preparation for summoning Ms Lewinsky to give evidence in secret court hearings.

The ruling ordering President Clinton's advisers to give evidence came on the same day that an open letter by Ms Lewinsky's lawyer, William Ginsburg, was revealed in a legal journal.

In it, Mr Ginsburg called for Kenneth Starr to be fired and he said that, at best, his case involved a consensual sexual relationship between his client and President Clinton.