Home-grown British terrorists now regard themselves as part of “global insurgency” that poses a new threat to international security, according to an official US report published yesterday.

The finding, in the State Department’s annual country report on terrorism, reflects growing fears on both sides of the Atlantic about radicalised young British Muslims.

British intelligence chiefs have confirmed to The Times that they are worried by the prospect of the next attack in America being committed by a British citizen arriving without hindrance in the US on the visa waiver programme. They are liaising closely with the FBI to identify possible suspects.

Sir David Manning, the British Ambassador to Washington, has said that the possibility of such an attack concerns him. However, he added that “the impulse of both countries has always been to work together closely on this issue” - otherwise there would have been drastic new restrictions on the ease of transatlantic travel by now.

Sir David cited his own experience after the bombings of July 7, 2005, in London, when he was immediately contacted by John Negroponte, then the head of US intelligence, saying: “Any help we needed we could have - and we got it.”

Yesterday’s State Department report referred to the plot to blow up airliners travelling between Britain and America. It stated: “Co-operation in derailing the transatlantic terror plot was evidence of the ability of the two governments to work together in real time to avert loss of life.” The report also quoted Dame Eliza Manningham-Buller, the head of MI5, who made the “stark assessment” that there were 200 known terror networks in Britain, 1,699 individuals and 30 terror plots. It said: “The British Government viewed radicalisation of young people in the UK and elsewhere ... as a growing threat to international and British security.”

The report said that there is evidence that al-Qaeda is shifting to “guerrilla terrorism”, through which it seeks to “manipulate and exploit grievances in at-risk populations”. It added: “A deeper trend is the shift in the nature of terrorism, from traditional international terrorism of the late 20th century into a form of transnational non-state warfare that resembles of form of global insurgency.”

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