The ruling, from a judge in Arkansas, effectively separates the latest inquiry - into whether Mr Clinton had an affair with a White House trainee, Monica Lewinsky - from the earlier sexual harassment case brought by Paula Jones, a former employee of the state of Arkansas. This case, in which Ms Jones claims that Mr Clinton made unwelcome sexual advances to her while he was governor, is set to come to trial in May and the pre-trial stage was due to be end by the end of January.
The much more recent investigation into allegations that Mr Clinton had a relationship with Ms Lewinsky and prevailed upon her to lie about it - allegations which exploded into public view only last week - were already threatening to run into the Paula Jones investigation. The possible duplication of effort - which could have had unpredictable effects on both cases - was the unintended result of the decision, taking by the US Attorney General, Janet Reno, to hand the Lewinsky inquiries to Kenneth Starr.
Mr Starr is the independent prosecutor originally appointed to handle the investigation into another area of Mr Clinton's activity, the Whitewater land deal in Arkansas. He was given jurisdiction in the Monica Lewinsky case for two reasons: first, because one of the main figures implicated in the Whitewater case, Mr Clinton's friend, Vernon Jordan, was also implicated in the Lewinsky case - it was he who is said to have found her a succession of jobs inside the White House.
The second reason was that crucial evidence was already in his hands. It was to him that Ms Lewinsky's confidante, Linda Tripp, had passed more than 20 hours of tape-recorded conversations in which she reportedly related details of her alleged affair with Mr Clinton. Even here, though, the two cases started to overlap awkwardly. The tape-recorded account reportedly conflicts with sworn testimony given by Ms Lewinsky to the Paula Jones inquiry. In that document, Ms Lewinsky had denied any affair.
Mr Starr's task was to establish whether the tapes or the sworn statement contained the truth. If it was the statement, then Ms Lewinsky was liable to be prosecuted for perjury. Something similar applied to Mr Clinton, who - if leaks are to be believed - also denied any affair with Ms Lewinsky in his sworn statement to the judge in the Paula Jones case.
Yesterday's decision followed a request from Mr Starr to the Arkansas judge, Susan Webber Wright, to halt all taking of evidence in the Paula Jones case. Within hours, Ms Webber Wright had issued her ruling, saying that evidence in the Lewinsky case would not be admissible in the Paula Jones. This means that Ms Jones's lawyers will not be able cite Ms Lewinsky's experience to corroborate any "pattern" of behaviour by Mr Clinton.
On the other hand, they may not need to. A report in the Washington Post yesterday suggested that another White House employee, Kathleen Willey, had already testified to Mr Clinton's mixing of sex and politics in the office.