The chief of Britain's top security agency has warned that Iran could be a nuclear power within two years.

Sir John Sawers of the MI6 agency told civil servants that the Iranians would have had nuclear weapons four years ago but for the efforts of British spies. He said MI6 agents were responsible for delaying "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".

In remarks disclosed by the Daily Telegraph, Sir John told a conference that the Ahmadinejad regime was "determinedly going down a path to master all aspects of nuclear weapons; all the technologies they need.

He said that it was evident that both Israel and the US would "face huge dangers" from a nuclear-armed Tehran and that both countries would have to make a decision about their response when, rather than if, the situation arises.

It comes the same week as a Pentagon report revealed evidence of advancements in Iran's weapons capability. The report warned that the country would "present a formidable force" in the event of a conflict, including with missiles that can reach Israel and Eastern Europe.