The BBC's Stephen Sackur in Washington: "This seriously undermines the impeachment proceedings"

The adviser, Sam Dash, said in a letter to Mr Starr: "You have violated your obligations under the independent counsel statute and have unlawfully intruded on the power of impeachment."

Sam Dash: Former Senate Watergate counsel

The former Senate Watergate counsel went on: "Against my strong advice, you decided to depart from your usual professional decision-making by accepting the invitation of the House Judiciary Committee to appear ... and serve as an aggressive advocate for the proposition that the evidence ... demonstrates that the president committed impeachable offences."

Mr Starr said he admired Mr Dash's decision.

"I think this country does well when people say I feel strongly about something and I'm going to stand up for it," Mr Starr said.

Kenneth Starr: "I admire his decision"

The special prosecutor was questioned by members of the House of Representatives Judiciary Committee for several hours on Thursday.

At the hearing, he gave no indication that he had a disagreement with Mr Dash.

Four-year working relationship

Mr Dash has been working for Mr Starr on a wide range of issues for the past four years, reviewing evidence and participating in the process on whether grand jury indictments should be sought.

Mr Dash, a registered Democrat, has been a lawyer for nearly 50 years and teaches at Georgetown University Law Centre.

In his letter to Mr Starr, he says that his resignation has got nothing to do with "many unfounded and misinformed attacks" on Mr Starr's conduct as an independent counsel.

He says he has been fully informed on all major decisions and mentions Mr Starr's willingness to be open to his advice.

Starr's lengthy cross-examination

During the first day of historic impeachment hearings in Washington, Mr Starr told the Judiciary Committee he publicly accused President Clinton of obstructing justice, lying under oath and tampering with witnesses.

David Kendall accused Mr Starr of an "overkill of investigation"

He said these were the issues, and not the President's relationship with Monica Lewinsky.

Democrats, however, criticised Mr Starr's relentless pursuit of Mr Clinton; one describing him as a sex policeman, and the White House issued a statement saying there was nothing new in Mr Starr's testimony.

President Clinton's lawyer David Kendall made several allegations of impropriety against Mr Starr's team, which he said had failed to turn up evidence of impeachable offences during its four-year inquiry.

He criticised the leaking of evidence to the media, and accused investigators of attempting to intimidate Monica Lewinsky.

But Mr Starr received the support of several Republicans on the committee, who complained that he had been pilloried by President Clinton's supporters.