MI6 agents have set back Iranian efforts to obtain nuclear weapons by at least six years, but Tehran will succeed in acquiring them by 2014, the head of the Secret Intelligence Service has warned.
At that point, the US and Israel will face a "very tough" decision over whether to launch a military strike against the Middle Eastern country, said Sir John Sawers.
Sir John's comments, reported in the Daily Telegraph, are the most public warning yet of the depth of concern within the security establishment about the nuclear ambitions of the regime of President Mahmoud Ahmadinejad.
Britain has been a driving force behind international sanctions designed to persuade Tehran to abandon hopes of nuclear weapons capability, and has also been part - alongside the US, Russia, China, France and Germany - of the E3+3 diplomatic offensive to find a peaceful resolution.
Speaking to an audience of senior civil servants in London, the MI6 chief made clear he did not believe Iranian claims that its nuclear programme is intended purely for civilian energy purposes.
"The Iranians are determinedly going down a path to master all aspects of nuclear weapons; all the technologies they need," said Sir John. "It's equally clear that Israel and the United States would face huge dangers if Iran were to become a nuclear weapon state."
He revealed that MI6 has "run a series of operations to ensure that the sanctions introduced internationally are implemented, and that we do everything we can within the Middle East to slow down these remaining problems".
Without the secret agency's actions "you'd have Iran as a nuclear weapons state in 2008 rather than still being two years away in 2012", said Sir John.
He said it was MI6's responsibility to "delay that awful moment when the politicians may have to take a decision between accepting a nuclear-armed Iran or launching a military strike against Iran". When that moment came, he said: "I think it will be very tough for any prime minister of Israel or president of the United States to accept a nuclear-armed Iran."
He added: "I take great pride in the fact that, in the last 10 years, over a number of jobs, I've been involved in an issue of global concern, and I feel that I as an individual (have made) an impact in the outcome of events."