North Korea is helping Iran to prepare an underground nuclear test similar to the one Pyongyang carried out last year.
Under the terms of a new understanding between the two countries, the North Koreans have agreed to share all the data and information they received from their successful test last October with Teheran's nuclear scientists.
North Korea provoked an international outcry when it successfully fired a bomb at a secret underground location and Western intelligence officials are convinced that Iran is working on its own weapons programme.
A senior European defence official told The Daily Telegraph that North Korea had invited a team of Iranian nuclear scientists to study the results of last October's underground test to assist Teheran's preparations to conduct its own — possibly by the end of this year.
There were unconfirmed reports at the time of the Korean firing that an Iranian team was present. Iranian military advisers regularly visit North Korea to participate in missile tests.
Now the long-standing military co-operation between the countries has been extended to nuclear issues.
As a result, senior western military officials are deeply concerned that the North Koreans' technical superiority will allow the Iranians to accelerate development of their own nuclear weapon.
"The Iranians are working closely with the North Koreans to study the results of last year's North Korean nuclear bomb test," said the European defence official.
"We have identified increased activity at all of Iran's nuclear facilities since the turn of the year," he said.
"All the indications are that the Iranians are working hard to prepare for their own underground nuclear test."
The disclosure of the nuclear co-operation between North Korea and Iran comes as Teheran seems set on a collision course with the West over its nuclear programme, although it insists it is entirely peaceful.
Both countries were named in President George W Bush's famous "axis of evil" State of the Union speech in 2002.
The United Nations Security Council has unanimously authorised the imposition of "smart" sanctions against Iran.
This is because of its refusal to suspend its uranium enrichment programme, which most Western intelligence agencies believe is part of a clandestine nuclear weapons programme.
France expressed concern yesterday over an Iranian decision to bar 38 UN nuclear inspectors from Iran, claiming that Teheran appeared to be singling out westerners from the inspection team.
Intelligence estimates vary about how long it could take Teheran to produce a nuclear warhead. But defence officials monitoring the growing co-operation between North Korea and Iran believe the Iranians could be in a position to test fire a low-grade device — less than half a kiloton — within 12 months.
The precise location of the Iranian test site is unknown, but is likely to be located in a mountainous region where it is difficult for spy satellites to pick up any unusual activity.
Teheran successfully concealed the existence of several key nuclear sites — including the controversial Natanz uranium enrichment complex — until their locations were disclosed by Iranian dissidents three years ago.
Western intelligence agencies have reported an increase in the number of North Korean and Iranian scientists travelling between the two countries.
The increased co-operation on nuclear issues began last November when a team of Iranian nuclear scientists met their North Korean counterparts to study the technical and political implications of Pyongyang's nuclear test.
The Iranians are reported to have been encouraged by the fact that no punitive action was taken against North Korea, despite the international outcry that greeted the underground firing.
This has persuaded the Iranian regime to press ahead with its own nuclear programme with the aim of testing a low-grade device, which would be difficult for international inspectors to detect.
Audio: Analysis of the developing crisis