History of a dubious sort will be made at the White House on Monday. For the first time, a sitting President will give testimony to a grand jury in a criminal investigation in which he is a suspect.

It is a humiliating position for President Clinton to be in, but he has little choice now but to go ahead. He is giving testimony "voluntarily" but only after he had been served with a subpoena by the relentless independent prosecutor, Kenneth Starr.

It will be a very tough day for the President as he will be questioned beginning at 1 p.m. for up to eight hours by skilled prosecutors who know far more about aspects of the investigation than he does, as they have already interviewed about 80 witnesses over the last six months.

Two concessions will be made to the President. He will be questioned in the White House instead of in the presence of the jury who will be in a courtroom about a mile away. The 23 jurors will hear and see the President on closed-circuit TV.

Unlike with other witnesses, the President will be allowed to have his personal lawyer, David Kendall, with him and he may be able to object to some questions but this is not certain.

Mr Clinton two weeks ago said publicly that he would give testimony "completely and truthfully". What the world will be waiting to see is if this testimony will contradict his sworn denial of last January of having had sexual relations with Ms Lewinsky when he testified to lawyers for Ms Paula Jones.

Some presidential advisers are floating the trial balloon that he may admit to some kind of sexual activity which would not be "sexual relations" as defined by Ms Jones's lawyers.

But if he continues to swear that he did not have sex with Ms Lewinsky, he will be at odds with her testimony of two weeks ago. It is still under seal but those familiar with it say that while she also denied last January that she had sexual relations with Mr Clinton, she is now saying that they did, including oral sex, over an 18-month period in the Oval Office.

According to legal sources, Ms Lewinsky also testified that she and the President discussed how they could conceal their affair but that he did not ask her to commit perjury in the civil action by Ms Paula Jones. This would be very helpful for the President as it could clear him of any charge of "suborning perjury".

When the President has testified and taken off for a badly-needed holiday on Martha's Vineyard, Mr Starr will finish writing his report on the investigation. If he believes there are "credible" grounds for believing that the President has committed "high crimes or misdemeanours" deserving of impeachment, he must send a report setting out the evidence to Congress.

It is widely believed that even without the President's testimony, Mr Starr is assembling his evidence and already has a first draft ready. But this does not mean that Mr Clinton is guilty of impeachable offences which remain ill-defined. Only the Senate can decide on his guilt or innocence by a two-thirds majority of its 100 members, and he must be presumed innocent until then.

It may be that Congress will not go ahead with the impeachment procedure. Having an adulterous affair - something Mr Clinton has up to now denied - would not be grounds for impeachment. That would not be a crime.

But if Mr Starr produces evidence that Mr Clinton obstructed justice or tried to get Ms Lewinsky to commit perjury then Congress could feel obliged to start the impeachment procedure, but perhaps next year after the November elections.

But most attention will focus on whether the President has committed perjury in swearing last January that he did not have sexual relations with Ms Lewinsky or whether he falls into a "perjury trap" on Monday when he will be questioned closely on this matter.

Mr Starr may believe he has overwhelming evidence that there was a sexual affair based on Ms Lewinsky's latest testimony, the 20 hours of taped conversations she had with former colleague, Linda Tripp, and masses of circumstantial evidence about the former intern's almost 40 visits to the White House, often at weekends and evenings.

Even more dangerous for the President is that Mr Starr almost certainly has the result of FBI laboratory tests on a blue dress handed over by Ms Lewinsky. As the whole world knows, she claimed that the dress had a stain which was evidence of an affair with the President.

Mr Clinton will be in the uncomfortable position of being asked if he stands over his denial of a sexual relationship without knowing the result of the DNA test on the dress, the details of what Ms Lewinsky has said in her latest testimony, what is on the Tripp tapes, and what many of the 70 witnesses have been telling the grand jury.

This is why Mr Kendall has been coaching the President for weeks over what may confront him on Monday and why Mrs Clinton has also been weighing in with her advice, embarrassing though this may be. The President's survival is at stake.

Whatever about lying last January in the Paula Jones civil action - and we outsiders cannot know that the President did - clear evidence that he lied under oath to the grand jury on Monday would put the President in legal jeopardy and probably mean the end of his presidency. But the evidence would have to be convincing to his supporters and not just his opponents. And we would not know this until Mr Starr has completed his report and Congress had released it or it was leaked.

It is not excluded that the President after his testimony will tell the nation what really happened between himself and Ms Lewinsky and ask for its understanding rather than wait for partisan leaks. Given the salacious details that have already emerged, this would be a huge ordeal for him while gripping television for the viewers. And it may be the only way of saving his presidency.

So Monday is perhaps the most important day in President Clinton's life.