The BBC's Stephen Sackur: "Polls now show Linda Tripp to be one of the most unpopular women in America."

After six hours of questioning, Ms Tripp left the US District Court House without making any comment.

Bill Clinton: denied sexual affair

Her lawyer, Anthony Zaccagnini, said that Ms Tripp said she "found it very easy to truthfully answer the questions posed to me by the prosecutor and the grand jury."

Ms Tripp's secretly-made tape recordings of a conversation in which Miss Lewinsky talked about sexual contacts she said she had with Mr Clinton form the basis of the current investigation that the President had sexual relations with Miss Lewinsky and then lied about it under oath.

Mr Clinton has denied having a sexual relationship with Miss Lewinsky.

Monica Lewinsky: pressure building up

Miss Tripp's evidence, which could last several days, is seen as a sign that the special prosecutor, Kenneth Starr, might be moving towards a conclusion of his inquiries.

A BBC correspondent in Washington says Mr Starr is also apparently trying to increase pressure on Miss Lewinsky herself to co-operate with him or face possible indictment for perjury.

Ms Tripp's testimony is being given in private to the grand jury examining the evidence against Mr Clinton. She is expected to give evidence again on Thursday.

Dale Young: testimony sparked debate

Another supposed friend of Monica Lewinsky, a businesswoman from New York, Dale Young, has already given evidence about what she was told.

Her testimony sparked a delicate debate in America about the definition of sexual relations.

Ms Young has said that President Clinton set limits and that, "nothing ever came to completion".