LONDON — North Korea is helping Iran to prepare an underground nuclear test similar to the one that Pyongyang carried out last year, according to a senior European defense official.

Under the terms of a new understanding between the two countries, the North Koreans have agreed to share with Tehran’s nuclear scientists all the data and information that they gathered from their successful test last October.

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 program.

A senior European defense official said that North Korea had invited a team of Iranian nuclear scientists to study the results of last October’s underground test to assist Tehran’s preparations to conduct its own — possibly by the end of this year.

At the time of the Korean firing, there were unconfirmed reports that an Iranian team was present. Iranian military advisers regularly visit North Korea to participate in missile tests.

Now, the long-standing military cooperation 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,” the European defense official said.

“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 U.N. Security Council has unanimously authorized the imposition of “smart” sanctions against Iran because of its refusal to suspend its uranium enrichment program, which most Western intelligence agencies believe is part of a clandestine nuclear weapons program.

France expressed concern yesterday over an Iranian decision to bar 38 U.N. nuclear inspectors from Iran, claiming that Tehran appeared to be singling out Westerners from the inspection team.

Intelligence estimates vary about how long it could take Tehran to produce a nuclear warhead. But defense officials monitoring the growing cooperation 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.