Tom Carver reports on the history of The Dress

The navy blue cocktail dress has become a legendary part of the alleged scandal.

In the early days of the crisis, Miss Lewinsky's colleague Linda Tripp told investigators that Miss Lewinsky had shown her a dress which she said had the president's semen on it. When the FBI searched Miss Lewinsky's flat they did not find it.

Linda Tripp: alerted investigators to the existence of the dress

It now transpires that she had sent it to her mother, Marcia Lewis, for safe keeping. It appears that Ms Lewis was granted the same legal immunity from prosecution as her daughter in return for handing the dress over for DNA analysis.

Investigators are also believed to have taken possession of several phone messages left by Mr Clinton on Miss Lewinsky's voice mail.

DNA specialist Dr Jay Siegel explains to Radio 5Live's Up All Night how the tests will work

Our correspondent says all this will help support Miss Lewinsky's testimony and make it harder for the president to deny that there was any sort of relationship between them.

But the FBI would not confirm that tests would include DNA analysis, nor did the spokeswoman indicate how long it might take for the tests to be completed.

The move follows President Clinton's decision to provide testimony to Mr Starr, to be videotaped at the White House next month.