Confidential intelligence documents obtained by The Times show that Iran is working on testing a key final component of a nuclear bomb.

The notes, from Iran’s most sensitive military nuclear project, describe a four-year plan to test a neutron initiator, the component of a nuclear bomb that triggers an explosion. Foreign intelligence agencies date them to early 2007, four years after Iran was thought to have suspended its weapons programme.

An Asian intelligence source last week confirmed to The Times that his country also believed that weapons work was being carried out as recently as 2007 — specifically, work on a neutron initiator.

The technical document describes the use of a neutron source, uranium deuteride, which independent experts confirm has no possible civilian or military use other than in a nuclear weapon. Uranium deuteride is the material used in Pakistan’s bomb, from where Iran obtained its blueprint.

“Although Iran might claim that this work is for civil purposes, there is no civil application,” said David Albright, a physicist and president of the Institute for Science and International Security in Washington, which has analysed hundreds of pages of documents related to the Iranian programme. “This is a very strong indicator of weapons work.”

The documents have been seen by intelligence agencies from several Western countries, including Britain. A senior source at the International Atomic Energy Agency (IAEA) confirmed that they had been passed to the UN’s nuclear watchdog.

A Foreign and Commonwealth Office spokeswoman said yesterday: “We do not comment on intelligence, but our concerns about Iran’s nuclear programme are clear. Obviously this document, if authentic, raises serious questions about Iran’s intentions.”

Responding to The Times’ findings, an Israeli government spokesperson said: “Israel is increasingly concerned about the state of the Iranian nuclear programme and the real intentions that may lie behind it.”

The revelation coincides with growing international concern about Iran’s nuclear programme. Tehran insists that it wants to build a civilian nuclear industry to generate power, but critics suspect that the regime is intent on diverting the technology to build an atomic bomb.

In September, Iran was forced to admit that it was constructing a secret uranium enrichment facility near the city of Qom. President Ahmadinejad then claimed that he wanted to build ten such sites. Over the weekend Manouchehr Mottaki, the Iranian Foreign Minister, said that Iran needed up to 15 nuclear power plants to meet its energy needs, despite the country’s huge oil and gas reserves.

Publication of the nuclear documents will increase pressure for tougher UN sanctions against Iran, which are due to be discussed this week. But the latest leaks in a long series of allegations against Iran will also be seized on by hawks in Israel and the US, who support a pre-emptive strike against Iranian nuclear facilities before the country can build its first warhead.

Mark Fitzpatrick, senior fellow for non-proliferation at the International Institute for Strategic Studies in London, said: “The most shattering conclusion is that, if this was an effort that began in 2007, it could be a casus belli. If Iran is working on weapons, it means there is no diplomatic solution.”

The Times had the documents, which were originally written in Farsi, translated into English and had the translation separately verified by two Farsi speakers. While much of the language is technical, it is clear that the Iranians are intent on concealing their nuclear military work behind legitimate civilian research.

The fallout could be explosive, especially in Washington, where it is likely to invite questions about President Obama’s groundbreaking outreach to Iran. The papers provide the first evidence which suggests that Iran has pursued weapons studies after 2003 and may actively be doing so today — if the four-year plan continued as envisaged.

A 2007 US National Intelligence Estimate concluded that weapons work was suspended in 2003 and officials said with “moderate confidence” that it had not resumed by mid-2007. Britain, Germany and France, however, believe that weapons work had already resumed by then.

Western intelligence sources say that by 2003 Iran had already assembled the technical know-how it needed to build a bomb, but had yet to complete the necessary testing to be sure such a device would work. Iran also lacked sufficient fissile material to fuel a bomb and still does — although it is technically capable of producing weapons-grade uranium should its leaders take the political decision to do so.

The documents detail a plan for tests to determine whether the device works — without detonating an explosion leaving traces of uranium detectable by the outside world. If such traces were found, they would be taken as irreversible evidence of Iran’s intention to become a nuclear-armed power.

Experts say that, if the 2007 date is correct, the documents are the strongest indicator yet of a continuing nuclear weapons programme in Iran. Iran has long denied a military dimension to its nuclear programme, claiming its nuclear activities are solely focused on the production of energy for civilian use.

Mr Fitzpatrick said: “Is this the smoking gun? That’s the question people should be asking. It looks like the smoking gun. This is smoking uranium.”