Cause for concern?

Does Tony Blair need to worry? More than one in three Labour backbenchers have signed a Commons motion opposing military action against Iraq; and there has been talk of plans to mount a challenge to his leadership of the party.

The short answer is that he is virtually impossible to remove as long as Labour remains in power, but that he needs to tread very carefully if Iraq is not to cause him serious trouble inside his party.

Suggestions of a leadership challenge recall what happened to Margaret Thatcher. In 1999 Sir Anthony Meyer's "stalking horse" candidacy attracted 33 votes. A year later Mrs Thatcher was out of office.

When Labour is in power, the only way to force a leadership election is for the annual conference to pass a resolution, on a card vote, that demands one. Today's would-be plotters must persuade most of the big unions and hundreds of constituency delegates to pass, in effect, a vote of no confidence in Mr Blair. Fat chance - for the moment.

However, if Britain takes part in a US-led invasion without the backing of the United Nations, then Mr Blair will face accusations from much of his party (and, indeed, a large slice of public opinion) that he is flouting international law.

Britain and America took great legal care when planning to eject Iraqi troops from Kuwait a decade ago.

The UN Security Council repeatedly instructed Saddam Hussein to withdraw his troops. On 29 November 1990, the council finally lost patience and approved resolution 678. This authorised Kuwait's allies "to use all necessary means" to liberate it, if Iraqi troops did not withdraw by 15 January. They did not, and Operation Desert Storm began. It was not universally popular, it was completely legal.

Likewise with last autumn's war to defeat the Taliban and the continuing campaign to wipe out al Qaeda. On 28 September, 17 days after the destruction of the World Trade Centre, Security Council resolution 1373, called upon member states to " cooperate to prevent and suppress terrorist attacks and take action against perpetrators of such acts".

Kosovo was slightly different. Nato started bombing Serbia in March 1999 without UN approval. However, Kofi Annan, the UN Secretary-General, "endorsed" the raids. This was a technical violation of the UN charter, but at a time when Slobodan Milosevic was engaged in a vicious act of ethnic cleansing.

Today's problems with Iraq are different. There is no specific emergency that cannot wait for a Security Council meeting. So can we be sure that Britain will send troops only if the UN approves? Foreign Secretary Jack Straw told Sir David Frost yesterday that we would act within international law, but said enigmatically that the Security Council need not be the only legal authority for action.

A few hours later, Jonathan Dimbleby pressed Defence Secretary Geoff Hoon on this point. Military action could be taken against Iraq without explicit UN approval, were this necessary "for selfdefence", said Mr Hoon.

This is the so-called Article 51 case for going to war. This section of the UN charter affirms "the inherent right of individual or collective selfdefence if an armed attack occurs against a member of the United Nations". So, if Saddam orders bombs to be dropped on London - or, indeed, any ally of ours - then we would have a clear right to retaliate.

That, though, was not what Mr Hoon was talking about. He said Britain could employ the "self-defence" gambit in order to combat the threat of military aggression by Iraq. Sorry, Mr Hoon, we can't.

The UN charter insists that an "armed attack" must actually take place. We are not allowed to get our retaliation in first.

Mr Straw said yesterday that Britain has "always" abided by international law during the 57 years since the UN was created. This is not true. We conspired with France and Israel to invade Egypt in 1956. The Suez crisis led to our biggest post-war humiliation and caused the downfall of a prime minister with a healthy majority. Mr Blair should have a higher ambition than to be a latter-day Sir Anthony Eden.