Britain’s intelligence services say that Iran has been secretly designing a nuclear warhead “since late 2004 or early 2005”, an assessment that suggests Tehran has embarked on the final steps towards acquiring nuclear weapons capability.

As world powers prepare to confront Iran on Thursday on its nuclear ambitions, the Financial Times has learnt that the UK now judges that Ayatollah Ali Khamenei, Iran’s supreme leader, ordered the resumption of the country’s weapons programme four years ago.

Iran is already under pressure after the revelation last week that it has been building an undeclared site to enrich uranium.

The UK’s assessment of Iran’s clandestine weapons programme will now add to concerns over Tehran’s capability, suggesting it could be making faster-than-expected progress on its nuclear project.

By contrast, US intelligence services remain firm in their conclusion that while Iran may ultimately want a bomb, the country halted weapons design work in 2003 and probably has not restarted that effort as of 2007.

The US published this judgment in a National Intelligence Estimate in 2007 amid claims that the CIA was scarred by its errors over Iraq’s weapons of mass destruction programme.

Britain has always privately expressed scepticism about the US assessment on Iran but is only now firmly asserting that the weapons programme restarted in 2004-05.

Iran’s chief nuclear official on Tuesday ruled out any discussions in Thursday’s talks with world powers over the country’s nuclear programme.

The comments by Ali-Akbar Salehi add to the pessimism that the talks in Geneva with the US, Britain, Germany, France, Russia and China will bear any fruit and will further fuel international suspicions about the link between Iran’s nuclear and missile plans.

“We will never bargain about our sovereign rights,” Mr Salehi said. “If we have the right to enrich uranium … convert uranium … have fuel fabrication … design reactors and manufacture reactors, we will do them and will not freeze them.”

He said Iran was committed to the “integrity” of the non-proliferation treaty and would not accept the six big powers’ “freeze-for-freeze” proposal, by which Iran should suspend all nuclear-related activities in return for a halt in international punitive measures. “They say the only guarantee you can give us is to stop all kinds of nuclear technologies and activities, but this is absolutely … nonsense,” Mr Salehi said.