In two boosts for his investigation, prosecutor Kenneth Starr has a dress from Monica Lewinsky that could be a key piece of evidence, legal sources say, and President Clinton has agreed to testify in the probe of an alleged presidential affair and cover-up.

The sources said attorneys for Lewinsky turned over a dress that she says was stained from an encounter with the president.One source familiar with the garment, speaking on condition of anonymity, said prosecutors likely would run forensic tests on the dress, examining it for evidence to corroborate Lewinsky's claim that she and Clinton had sexual relations.

The sources provided several new details of Lewinsky's proffered testimony, saying she has told prosecutors she discussed with the president the possibility of returning gifts he gave her to presidential secretary Betty Currie.

The sources said Lewinsky would testify that she and Clinton agreed that the gifts, including a book, should be given to Currie and that the testimony would help further prosecutors' investigation of possible obstruction of justice.

Lewinsky gave the gifts to Currie after she received a subpoena in December from lawyers in the Paula Jones sexual harassment lawsuit against Clinton that sought her testimony and evidence, sources have said in the past.

In the other major development Wednesday, Clinton agreed to break six months of silence and provide videotaped testimony concerning his relations with Lewinsky, a former White House intern who claims she had an 18-month sexual relationship with Clinton beginning in 1995.

Under an agreement with Starr, Clinton will answer questions Aug. 17 from inside the White House with his lawyers present.

Senior aides said Clinton is prepared to repeat his denial of any sexual relations with Lewinsky. If so, his testimony could conflict with the former intern. Legal sources, speaking on condition of anonymity, say she told prosecutors this week she and the president had sexual relations and discussed ways of concealing it.

Clinton's arrangement with Starr averted an unprecedented presidential appearance under subpoena before a federal grand jury where witnesses appear without counsel. Starr agreed to withdraw the subpoena to accommodate Clinton, and the president will delay his summer vacation to testify. His testimony, under oath, will be made available to the grand jury.

Regarding the dress, Lewinsky's lawyers, Jacob Stein and Plato Cacheris, offered the garment to Starr's office as part of the deal in which Lewinsky gained immunity from prosecution, the sources said. Lewinsky's first lawyer, William Ginsburg, did not proffer the dress to prosecutors, according to the legal sources. Ginsburg repeated Wednesday night that he knows nothing about any such clothing.

Lewinsky, 25, showed friend Linda Tripp in late October or early November a dark dress that the former intern said had been stained by Clinton during sex, the legal sources said. The dress shown to Tripp was in Lewinsky's apartment closet at the Watergate.

According to one source, the dress was moved from the apartment before it was searched by the FBI when the investigation got under way in January.

Senate Judiciary Chairman Orrin Hatch, R-Utah, said Starr apparently is examining Clinton for three possible crimes - perjury, causing someone else to commit perjury and obstruction of justice.

"We're now in the closing throes of this," Hatch said. "It's in everybody's interest to get this over with."

"Agreeing to allow testimony by videotape was a good concession by Ken Starr," Hatch told the Deseret News.

"This thing will never be cleared up - and Starr would not be able to issue a report to Congress - until the president and Miss Lewinsky testify. Now, I think we are getting to the end of this thing," Hatch said.

Hatch added, "My advice to the president would be: Just tell the truth. The American people are understanding if you tell the truth. But if you don't tell the full truth, they resent it."

Hatch has been among the loudest voices in Congress calling on Clinton for months to end his silence about the matter and fully explain it to investigators and the public.

Hatch even single-handedly revived nationally an old word - "canoodle" - to attack Clinton, saying if allegations about the affair are true then Clinton would go down as the "greatest canoodler in history."

While that little-known word may spark wild imagination, it actually means to neck, kiss or caress. Hatch said he and his staff "searched it out because I was looking for a word that is milder than `adulterer' or `fornicator."'