Downing Street has said that Iran's nuclear ambitions remain a "very serious issue" despite a US intelligence report finding no evidence of a weapons-building programme.

Number 10 was responding on Tuesday to a new National Intelligence Estimate in Washington which found that Tehran stopped its weapons programme in 2003.

Iran has welcomed the findings, which have caused controversy in America, as a victory. But Britain backed the White House's continuing concern.

"We need to examine the detail of this," the prime minister's spokesman said.

"But in broad terms the government believes it confirms that we were right to be worried about Iran seeking to develop nuclear weapons, it shows the sanction programme and international pressure had an effect and it tells us that the intent was there and the risk of Iran acquiring a nuclear weapon remains a very serious issue."

The spokesman also rejected a suggestion that a complete distinction could be drawn between Iran's civil nuclear power programme and the risk of it developing weapons.

"Iran's civil nuclear programme is continuing and once it masters the technology to enrich uranium for civilian purposes it can readily use the same technology to produce weapons-grade uranium," he said.

He added that Tehran had not engaged fully with the International Atomic Energy Authority inspectorate and that Britain would be seeking further talks "with our key international allies and will be looking to discuss it with the UN".

Earlier shadow foreign secretary William Hague told BBC Radio 4's Today programme that the report was significant but "rather ambiguous in many ways".

"It would be a great mistake for the international community to say 'oh, thank heavens for that, we don't have to worry about this at all'," he said.

"Iran is continuing in defiance of two [UN] security council resolutions, continuing to build centrifuges and to enrich uranium.

"This report says it could develop a nuclear weapon from 2010 onwards. So this problem has not gone away. It has perhaps changed in its nature with this report but it has not gone away."