"There have been a number of incidences of successful prosecutions of individuals going to prepare terrorism. We should seek to use those first before throwing away very important legal principles."
In his article for The Telegraph, the Mayor of London also joins calls for jihadists to be stripped of their citizenship, despite opposition from Theresa May, the Home Secretary, who warned at the weekend that such a move would be illegal.
He calls for control orders, which kept terrorism suspects in their homes, to be brought back amid concerns that hundreds of jihadists could return to Britain and pose a threat to national security if Isil (the Islamic State in Iraq and the Levant) loses ground in Iraq and Syria.
Mr Johnson’s intervention came as the British ambassador to the US said intelligence agents believe they have identified “Jihadi John”, the Briton responsible for beheading the American journalist James Foley, after employing voice recognition technology.
Sir Peter Westmacott, the ambassador to Washington, also disclosed that 70 militants have been arrested after returning from Syria, a number of them carrying instructions for “very specific missions” to unleash terrorist atrocities on British soil.
Mr Johnson says Britain needs to help to “close down” the Islamic caliphate before it is too late, adding that “doing nothing is surely worst of all”.
He says: “If we let Isil get their way, then we will be acquiescing, first, in a gigantic and violent change in international borders.
“Next, we will be allowing a new and hideous regime to be born, a country where black-flag waving jihadis compete to show they have the most bigoted and reactionary understanding of their religion by persecuting women, Jews, Christians, gays, Yazidis aide Shi’ites.
“The place would be a giant training ground for terrorists and wannabe jihadis. We need to try to close it down now, before it gets worse.”