Travel restrictions could be imposed by America on 800,000 ?British? citizens of Pakistani origin because of concerns about terrorism, it emerged yesterday.  The move has been prompted by fears that British Muslim men were behind several major bomb plots.  On Monday, five men - four with Pakistani backgrounds - were convicted of plotting to attack a shopping centre, a nightclub and other targets with home-made fertiliser bombs. Last summer, MI5 foiled an alleged plot for suicide attacks on transatlantic aircraft.  The Americans have been concerned that their principal security risk could actually come from Britain, with whom they operate a visa waiver scheme. Since 2004, Britons travelling to the US do not need a visa if they have a machine-readable passport issued since 1991.  Michael Chertoff, the US homeland security secretary, has reportedly told the Government that British Pakistanis should apply for a visa before travelling to America.  The Foreign Office said last night any such plan would be resisted although the Americans would be entitled to introduce a new visa regime unilaterally.  Last month, Mr Chertoff said that America was vulnerable to Muslim citizens of Britain and other European countries mounting terrorist attacks. Terrorists such as the four July 7 bombers - three of whom were of Pakistani origin - could have used the waiver scheme to enter the US.  We need to build layers of protection and I dont think we totally want to rely upon the fact that a foreign government is going to know that one of their citizens is suspicious and is going to be coming here, he said. Bruce Riedel, senior fellow at the American think-tank the Brookings Institution, echoed his concerns.   Omar Khyam, the leader of the plotters convicted on Monday, was, he said, the classic UK-Pakistani connection that al-Qaeda has focused on since 9/11. His UK passport gives him international nobility. His training at a camp run for Kashmiris by Pakistans Inter-Services Intelligence agency gives him expertise. Al-Qaeda gives him direction.  An estimated 400,000 trips a year are made by Britons of Pakistani origin to their ancestral homeland.