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 program.
An Asian intelligence source last week confirmed 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.
The documents have been seen by intelligence agencies from several Western countries. A senior source at the International Atomic Energy Agency confirmed they had been passed to the UN's nuclear watchdog.
A British Foreign and Commonwealth Office spokeswoman said yesterday: "We do not comment on intelligence, but our concerns about Iran's nuclear program are clear. Obviously this document, if authentic, raises serious questions about Iran's intentions."
Responding to the findings, an Israeli government spokesperson said: "Israel is increasingly concerned about the state of the Iranian nuclear program and the real intentions that may lie behind it."
The revelation coincides with growing international concern about Iran's nuclear program. Tehran insists it wants to build a civilian nuclear industry to generate power, but critics suspect the regime is intent on diverting 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 Mahmoud Ahmadinejad then claimed that he wanted to build 10 such sites. At the weekend Iranian Foreign Minister Manouchehr Mottaki said 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 (justification for acts of war). If Iran is working on weapons, it means there is no diplomatic solution."
The Times newspaper obtained the documents, which were written in Farsi, and had them translated into English -- and had the translation separately verified by two Farsi speakers. While much of the language is technical, it is clear 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 Barack Obama's groundbreaking outreach to Iran. The papers provide the first evidence that suggests Iran pursued weapons studies after 2003 and may actively be doing so today. 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 resumed by then.
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."