The most immediate impact will be in the region itself. Influential circles in Tokyo are calling for a debate on whether the country should reverse its anti-nuclear policy, formed after the Hiroshima and Nagasaki bombings, and build a nuclear deterrent.

Similarly, Seoul could develop its own programme if it felt that its existence was threatened by the North, with which it is still technically at war.

Mark Fitzpatrick, an expert on nuclear proliferation at the International Institute for Strategic Studies, said that while America provided a defensive shield it was unlikely that either state would pursue a nuclear military option. But he added: “This test could have a ripple effect in some unlikely places.”

British officials said that the Iranian regime would be carefully studying how the international community will respond. Iran is thought to be only a few years away from acquiring the technology to build its own atomic device.

Other Middle Eastern countries would be tempted to develop a deterrent. Turkey is embarking on a civilian nuclear programme, and Egypt is openly debating whether it should go the same way.

Arguably one of the most dangerous aspects of North Korea joining the nuclear club is the prospect that it will export its know-how. Michael Yahuda, of the London School of Economics, said there was a danger that it could sell nuclear weapons to terrorists or other “rogue” states.

Underpinning the concern about North Korea’s test is the threat to the NPT, which for nearly 40 years has allowed the peaceful development of nuclear power while limiting the spread of weapons. Apart from the five countries permitted to have nuclear weapons — America, Britain, China, France and Russia — only three have built atomic weapons: Israel, India and Pakistan, none signatories of the NPT. North Korea withdrew from the treaty three years ago. If others follow the world could face a new nuclear arms race.

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