We should not bomb Iran to prevent Iran getting the bomb. The consequences would be disastrous. After Iraq, US or Israeli military action against this regionally powerful, oil-producing Shia muslim country would make the world a still more dangerous place. The cure would be worse than the disease. That's what a new report from a diverse coalition of British organisations says, and it is right. But this is not enough. joining with wiser heads in Washington to prevent George Bush making a final gung-ho blunder is only a preliminary to the real business. Anyone who, after a bracing afternoon walk chanting "stop the war" and "stop Bush", goes home thinking they have made the world a safer place needs to think some more.

If we don't bomb Iran, Iran is quite likely to get the bomb. If Iran gets the bomb, Saudi Arabia, Egypt and others in the Middle East will be tempted to follow. The last barriers to nuclear proliferation, already breached by North Korea, Pakistan, India and Israel, could rapidly break - in the most volatile region in the world. The risk of nuclear war will then be greater than it was in the 1980s, when CND, END and other west European peace movements marched against new US and Soviet missile deployments. The likely scale of the nuclear conflict is much smaller than a superpower nuclear apocalypse, but that in itself makes it more not less probable that an unhinged leader would take the risk.

On the available evidence, the Islamic Republic of Iran is trying to edge towards a technological position from which it could, should it choose, rapidly move towards 90% uranium enrichment and the production of nuclear weapons. The best analysis we have suggests that Ayatollah Khameini, the supreme leader of the revolutionary regime, has not made a decision to go for nuclear weapons, and it would take a number of years to get there even if he had. But Iran has been doing a number of things that are not explicable simply by a desire to have the civilian nuclear energy to which it is entitled under the nuclear non-proliferation treaty.

The real question is therefore how, without the use of force, you can stop Iran going down this path. This requires pressure as well as incentives. In 2003, when the Islamic Republic felt itself weak, with a low oil price squeezing its budget and (yes) the unsettling spectacle of a US occupation of Iraq next door, it was more ready to negotiate on the nuclear issue. Last year, when it felt itself strong, with a high oil price gorging its budget, President Ahmadinejad riding high on a populist wave, and Iran rather than the US increasingly calling the shots in the politics of Iraq, it turned down the best offer it had received since the last year of the Clinton administration.

All it needed to do was to suspend uranium enrichment and the US secretary of state, Condoleezza Rice, would have personally joined talks with Iran - something not seen from a senior US official since the Iranian revolution nearly 30 years ago. Last summer's "Vienna proposals" of the five permanent members of the UN security council plus Germany (the P5 plus 1), which I have before me as I write, offered to support the building of light-water reactors in Iran and to guarantee a supply of nuclear fuel enriched in Russia. The political and economic incentives were vaguer, but they included supporting Iran's full integration into the World Trade Organisation and a trade agreement with the EU, as well as possible deals on civil aviation, hi tech and telecommunications. And that was just the opening offer.

After some haggling worthy of its main bazaar, Tehran said no. Following a complex diplomatic dance with Russia and China, the UN security council passed a resolution just before Christmas which imposed rather minimal sanctions. Later this month, we hit the 60-day deadline for the UN to review whether Iran has complied with the resolution, which calls for the suspension of "all enrichment-related and reprocessing activities".

If it complies, the direct negotiation can begin. If it does not, the indirect negotiation continues. Either way, we need two plans. Plan A involves mustering every peaceful instrument at our disposal to steer the Iranian regime away from its current course. We have not yet done everything we can. Broadly speaking, the US needs to offer more carrots, the EU needs to brandish more sticks. As the Baker-Hamilton commission and many US foreign policy sages have urged, the US should open bilateral talks with Iran, without conditions. It should be prepared, longer term, to offer a "grand bargain", in which it restores the full panoply of normal diplomatic and economic relations, provided Iran desists from developing nuclear weapons and (more tricky to judge and verify) supporting terrorists. We should also establish an impartial, UN-supervised system of supplying nuclear fuel for civilian purposes.

But carrots are not enough. This also needs sticks. If the military sticks are to be taken off the table, what remains are economic ones - and those are in the hands of the Europeans. Because of history and its own bilateral sanctions, the US does very little business with Iran; Europe does lots. Even if we think that economic sanctions would, in the long run, be counter-productive, we in Europe must be prepared credibly to threaten them. Since we already live in a multipolar world, we would still have a big problem bringing an undemocratic China, hungry for Iranian oil, and a bolshy Russia, on to the same course, but the buck starts here.

Beyond this, we need to recognise that Iran has both a complex, far from monolithic political system and a young, critical society. Ahmadinejad is not Iran. With the oil price down to around $50 a barrel, western credits and foreign investment drying up, inflation rising and Saudi Arabia flexing its muscles as Sunni-Shia tensions mount across the region, his government is no longer riding so high.

In local elections last December, his candidate list, wonderfully named The Pleasant Scent of Service, was blown a raspberry. Before every step, we need first to ask this question: how will it affect the dynamics of the regime and the society? We need a skilful public diplomacy, media innovations like the BBC's new Farsi-language television service, people-to-people contacts, and a hundred other initiatives to inform and to open up Iranian society. Europe has not begun to unfold its full potential in this regard. The effects will only be apparent over a number of years, but it may be a number of years before Iran is within striking distance of making nuclear weapons.

And Plan B? If all this fails, and we're not going to bomb Iran, then Plan B can only be containment and deterrence. The price to Iran of testing, let alone actually using, a nuclear device should be set very high. We should start now taking all measures we can to prevent an Iranian bomb being swiftly followed by a Saudi or Egyptian one. But I wouldn't count on this working either.

So here's the score: if we bomb Iran, the world will be a more dangerous place. If Iran gets the bomb, the world will be a more dangerous place. Conclusion: the world is likely to be a more dangerous place.