On Wednesday, the Senate issued subpoenas for depositions from former White House intern Monica Lewinsky, Mr Clinton's friend Vernon Jordan and a White House aide, Sidney Blumenthal.

The BBC's Clive Myrie: President Clinton has made it clear he will never testify

The witnesses will be interviewed in private. A further vote will be needed for them to be publicly questioned.

Prosecutors have also formally asked the Senate to invite President Clinton himself to appear at a deposition, but he has ruled that out.

Listen to hightlights from Tuesday's hearing in this report from Paul Reynolds in Washington

One of the managers handling proceedings, Rep Bill McCollum, promised that Ms Lewinsky would not be asked for explicit sexual details of her affair with the president.

He also said it was exceedingly important that Mr Clinton made himself available for questioning.

"The two people who know the most about this are Monica Lewinsky and President William Jefferson Clinton," Mr McCollum said.

President Clinton's lawyer David Kendall had ridiculed the call for witnesses, comparing the managers to a dithering character in a novel by Charles Dickens.

"The house managers are like the character in David Copperfield, Mr Micawber, who was always hoping that something would turn up.

"They continue to hope that something will turn up for them. They don't know what it is, but they believe they'll know it when they see it."

Witness list trimmed

The BBC's Stephen Sackur in Washington: "The trial is at a turning point"

The prosecutors cut their witness list down to the minimum, deciding against calling Mr Clinton's secretary Betty Currie, among others.

"They have told us the shorter the list, the more likely an affirmative vote," lead prosecutor Henry Hyde told reporters.

"We have to be realistic. It's not our call, it's their call."

Mr Jordan, a long-time friend of the President, helped find Ms Lewinsky a job around the time she emerged as a subpoenaed figure in the Paula Jones sexual harassment case.

The BBC's Tom Carver in Washington: No chance of Clinton testifying

Mr Jordan denies the job search and the lawsuit were connected.

Mr Blumenthal was one of the aides to whom President Clinton first denied having a sexual relationship with Ms Lewinsky after the controversy erupted.

President Clinton was in St Louis on Tuesday, meeting Pope John Paul II.

White House warns of delay

White House Press Secretary Joe Lockhart has warned that the decision to call witnesses will require delays to allow presidential lawyers time for their own interviews and investigations.

"No matter how much you slim down the witness list there is still going to be delay," Mr Lockhart said.

Ms Lewinsky decided on Tuesday to leave Washington, where she had been interviewed by the House prosecutors.

Her lawyer, Plato Cacheris said: "She and I hope that she will not have to testify. If she's needed, they issue a proper document to get her back here, she will return."

Ms Lewinsky's lawyers have said she had told the prosecutors nothing that was not already on record.

A new opinion poll, meanwhile, shows two-thirds of Americans want the Senate to end the trial immediately.