Sir John Sawers, the head of MI6, the external intelligence service, has made an extraordinary secret visit to Israel to urge Prime Minister Benjamin Netanyahu not to give the go-ahead to plans to bomb Iran, the Daily Mail newspaper revealed Wednesday. In an indication that the UK Government believes a strike on Tehran's nuclear programme could be imminent, Sir John Sawers is understood to have made a personal mission to deliver a clear message that Britain is opposed to action now. It is unusual for the head of MI6, who is known in government circles as 'C', to make a foreign visit as an emissary of the Government, and still more so for details to leak, the popular newspaper noted. British Prime Minister David Cameron is understood to have become increasingly concerned at the rhetoric from the authorities in Israel, who have been threatening unilateral military action to halt Iran's nuclear drive. Netanyahu and his defence minister Ehud Barak have been pushing London and Washington to take a tougher line against Iran amid growing concern about suspected nuclear sites, the Mail reported. The issue has been discussed by the Government's National Security Council, with some observers suggesting Israel may seek to strike before the US presidential elections in November, while the leadership in Washington is in a state of limbo. British fears about an imminent strike on Iranian nuclear facilities are said to have been heightened by Netanyahu's failure to provide clear answers to questions about Israel's intentions. Sir John is thought to have met Netanyahu and Barak, as well as a number of Israeli security and diplomatic officials. He stressed that Britain believes there is still time for diplomatic and economic sanctions against Iran to succeed in deterring its hardline regime from developing a nuclear weapon. Britain, the US and Israel have long suspected that Tehran is trying to develop a nuclear bomb, in flagrant contravention of the Nuclear Non-Proliferation Treaty. Tehran has always denied any military motivation for the programme, claiming it is for civilian purposes. Foreign Office officials say there are signs that the latest sanctions, imposed in July to bar Iranian oil exports to the EU, are having a 'serious impact' on the Islamic republic. The mission, thought to have taken place around two weeks ago, has failed to cool Israeli rhetoric, the paper added. In comments which appeared to bring the possibility of an attack closer, Netanyahu last night took the West to task for failing to act more decisively. Last Monday, US Secretary of State Hillary Clinton said Washington would not set a deadline in further talks with Iran, saying there was still time for diplomacy to work. US Defence Secretary Leon Panetta has said that Washington would have little more than a year to act to stop Tehran if it decided to produce a nuclear weapon. Israel's "Haaretz" daily newspaper was the first media outlet to reveal yesterday that the UK had sent a special envoy to Israel in a move designed to urge Tel Aviv not to attack Iran. But the paper did not identify the envoy.