Now that the US president has announced that he has not ruled out an attack on Iran, if it does not abandon its nuclear programme, the Middle East faces a crisis that could dwarf even the dangers arising from the war in Iraq.

Even a conventional weapon fired at a nuclear research centre - whether or not a bomb was being made there - would almost certainly release radioactivity into the atmosphere, with consequences seen worldwide as a mini-Hiroshima.

We would be told that it had been done to uphold the principles of the nuclear non-proliferation treaty (NPT) - an argument that does not stand up to a moment's examination.

The moral and legal basis of the NPT convention, which the International Atomic Energy Agency is there to uphold, was based on the agreement of non-nuclear nations not to acquire nuclear weapons if nuclear powers undertook not to extend nuclear arsenals and negotiate to secure their abolition.

Since then, the Americans have launched a programme that would allow them to use nuclear weapons in space, nuclear bunker-busting bombs are being developed, and depleted uranium has been used in Iraq - all of which are clear breaches of the NPT. Israel, which has a massive nuclear weapons programme, is accepted as a close ally of the US, which still arms and funds it.

Even those who are opposed, as I am, to nuclear weapons in every country including Iran, North Korea, Britain and the US, accept that nuclear power for electricity generation need not necessarily lead to the acquisition of the bomb.

Indeed, many years ago, when the shah - who had been put on the throne by the US - was in power in Iran, enormous pressure was put on me, as secretary of state for energy, to agree to sell nuclear power stations to him. That pressure came from the Atomic Energy Authority, in conjunction with Westinghouse, who were anxious to promote their own design of reactor.

It is easy to understand why president Bush might see the bombing of Iran as a way to regain some of the political credibility he has lost as a result of the growing hostility in America to the Iraq war due to the heavy casualties suffered by US forces there .

It is inconceivable that the White House can be contemplating an invasion of Iran, and what must be intended is a US airstrike, or airstrikes, on Iranian nuclear installations, comparable to Israel's bombing of Iraq in 1981. Israel has publicly hinted that it might do the same again to prevent Iran developing nuclear nuclear weapons.

Such an attack, whether by the US or Israel, would be in breach of the UN Charter, as was the invasion of Iraq. But neither Bush, Sharon nor Blair would take any notice of that.

Some influential Americans appear to be convinced that the US will attack Iran. Whether they are right or not, the build-up to a new war is taking exactly the same form as it did in 2002. First we are being told that Iran poses a military threat, because it may be developing nuclear weapons. We are assured that the President is hoping that diplomacy might succeed through the European negotiations which have been in progress for some months.

This is just what we were told when Hans Blix was in Baghdad talking to Saddam on behalf of the UN, but we now know, from a Downing Street memorandum leaked some months ago, that the decision to invade had been taken long before that.

That may be the position now, and I fear that if a US attack does take place, the prime minister will give it his full support. And one of his reasons for doing so will be the same as in Iraq: namely the fear that, if he alienates Bush, Britain's so-called independent deterrent might be taken away. For, as I also learned when I was energy secretary, Britain is entirely dependent on the US for the supply of our Trident warheads and associated technology. They cannot even be targeted unless the US switches on its global satellite system.

Therefore Britain could be assisting America to commit an act of aggression under the UN Charter, which could risk a major nuclear disaster, and doing so supposedly to prevent nuclear proliferation, with the real motive of making it possible for us to continue to break the NPT in alliance with America.

The irony is that we might be told that Britain must support Bush, yet again, because of the threat of weapons of mass destruction, thus allowing him to kill even more innocent civilians.

· Tony Benn will be talking about War; Religion and politics; and Democracy, at the Shaw Theatre in London on September 7, 8 and 9

Tony@tbenn.fsnet.co.uk