Sir John said it was up British intelligence 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.”

“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.” Without previous efforts by British intelligence, he was quoted as saying, “you’d have Iran as a nuclear weapons state in 2008 rather than still being two years away in 2012.” He did not elaborate.

In recent years, several Iranian scientists have been assassinated on the streets of Tehran and a computer virus called Stuxnet has disrupted computer systems at nuclear facilities in Iran. Tehran has accused the American, Israeli and British secret services of conducting covert operations against it.

Sir John said 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.”

Earlier this month, low-level talks between Iran and the group of big powers over the Iranian nuclear program ended early with both sides saying the deputies of their top negotiators would meet at a later date. Their announcement gave no hint of progress but nonetheless suggested that neither side was ready to declare the effort a failure.

The talks, in Istanbul, were part of a series of negotiations this year and were held against a backdrop of increasingly bellicose oratory by Iran and the United States because of the nuclear impasse, which has started to raise tensions again in the Middle East.

Iran has renewed a threat to close the Strait of Hormuz, a vital Persian Gulf oil conduit, in response to intensified American and European sanctions meant to paralyze the Iranian oil industry as a pressure tactic in the nuclear talks. Iran has also said the new sanctions will have no effect on its resolve to prevail in the nuclear dispute.