Secretary of State Colin L. Powell's presentation of the U.S. case against Iraq on Wednesday could mark a turning point in the Bush administration's efforts to convince the U.N. Security Council that military force is justified, but only if he produces significant new evidence and particularly if it details active ties between the Iraqi government and al Qaeda, according to council diplomats.

The diplomats, from a range of governments that oppose or support the U.S. position, or are warily sitting on the fence, said that if Powell offers credible proof beyond, as one envoy put it, "affirmations and statements" of previously disclosed information, it could have a significant impact on international public opinion, particularly in Europe.

The administration is still debating whether to release some of its most sensitive intelligence, which U.S. officials have said includes electronic intercepts of Iraqi officials allegedly plotting to impersonate weapons scientists and hide documents from U.N. inspectors, as well as photographs of weapons materials being moved and concealed. The intelligence also includes information about al Qaeda links that has come from Iraqi defectors and war detainees, the officials said.

"A lot of people have never seen it and just don't know," said a senior administration official, who added that the information has been shared only with Britain. "The French don't have a clue about this. . . . The accumulated mass of evidence is very, very overwhelming." Other officials, however, have said much of the information is dated and circumstantial.

Even if Powell reveals convincing proof about Iraq's violations of its disarmament commitments or about its ties with al Qaeda, however, it is far from clear that the Security Council will endorse the Bush administration's plans to confront Iraq by force. "Even if one believes that the chances of successfully cleaning up this mess in Iraq are not very good, the overriding necessity to wage war is not being made clear," said one European diplomat, reflecting the view of a number of council members.

The debate over what to do about Iraq, and when to do it, has become increasingly rancorous in recent weeks. The administration and other council members led by France, Russia and Germany have come close to accusing each other of bad faith in agreeing to last November's Security Council resolution giving Baghdad "a final opportunity" to comply with disarmament demands and warning of "serious consequences" if it balks.

Many in the administration said they believe the countries objecting to declaring the two-month-old inspections a failure and moving to what it sees as the previously agreed next step -- a military confrontation -- never really intended to go to war, no matter how obvious it became that Baghdad was not cooperating.

But many council members say that accusation "can play both ways," another diplomat said. Perhaps, he said, President Bush went to the United Nations only to seek its endorsement of confronting Iraq over weapons of mass destruction to give the impression he preferred a diplomatic solution. Perhaps he was never really interested in the ideas of other Security Council members. As this envoy put it, many now believe Bush planned from the start to create a situation in which "the Security Council's only choice was to support a war or be declared irrelevant."

The administration has never made a secret of its belief that it has the right to act unilaterally, or with what Bush calls a "coalition of the willing," against Iraq, without additional U.N. approval. Other council members do not dispute that the Nov. 8 resolution left room for widely divergent interpretations. "It papers over a significant difference of opinion on what constitutes a mandate to use force and what does not," said another European diplomat.

But council diplomats who question what they see as the U.S. rush to war, even if Iraqi President Saddam Hussein is defying the inspectors, said they find the U.S. arguments disingenuous. "Saddam Hussein is a nasty guy. Everybody knows that," said a non-European council ambassador. "We knew it before [Resolution] 1441. That was the point of the resolution in the first place." Unless and until the inspectors come to the council and declare it hopeless to continue their work, "what's the reason to call off inspections?" he asked.

In talks with Bush at the White House on Friday, British Prime Minister Tony Blair counseled a delay of as much as six weeks before declaring the inspections over. Blair, who is under heavy domestic pressure not to participate in an American-led war waged without U.N. approval, believes council agreement is still possible, according to officials.

At a news conference with Blair on Friday, Bush said he wants the debate over Iraq resolved in the next several weeks and will not allow the Security Council to drag it out. According to officials, Blair's position is that a few additional negative assessments of Iraqi cooperation from the U.N. weapons inspectors, such as the one delivered to the council last Monday, along with Powell's new evidence, could convince most members there is little point in continuing the inspections.

It may never be possible to win the votes of France, China and Russia, each of which has the power to veto a new resolution authorizing military force. But Blair and other council members believe the three countries might eventually be willing at least to abstain from a resolution declaring that Iraq has violated Resolution 1441, as long as it did not specifically authorize military action.

In the administration's view, that would be tantamount to agreement on the use of force. In its opening paragraphs, Resolution 1441 declares that Iraq is still in violation of Security Council Resolution 678, adopted in 1990, which called for the use of "all necessary means" in authorizing the Persian Gulf War. It is a tenuous and somewhat bureaucratic argument. But it is one that a number of council diplomats -- even those opposed to new military action -- agreed would likely not be challenged and could provide their governments with political cover at home.

Any hope of an agreement, a European diplomat said, rests with Powell. He is widely trusted by council governments, and many said his words this week will have a heavy impact.

"You are lucky to have a representative for this administration that is as credible as he is," the European diplomat said. "If you didn't have him, you'd really have much, much greater difficulties working with a whole lot of Europeans."

Staff writer Glenn Kessler contributed to this report.

Secretary of State Colin L. Powell will present evidence against Iraq to the Security Council on Wednesday.British Prime Minister Tony Blair has suggested waiting for additional assessments from inspectors.