Founded quietly in 2003, then expanded after the 7/7 London bombings, which killed 52 people, Prevent started on a small scale. But a rising drumbeat of terror attacks over the next decade prompted calls for expansion.

After the murder of fusilier Lee Rigby outside an army barracks in south-east London in 2013, prime minister David Cameron told parliament: “When young men born and bred in this country are radicalised and turned into killers . . . we have to do more.”

What emerged was a tougher approach to pre-criminal extremism than in perhaps any other country. New legislation requires public officials working in schools, universities, hospitals and local councils to report on individuals showing radical tendencies.

The Home Office claims that, so far, the programme has successfully diverted more than 1,200 people from extremism and, potentially, acts of terror. After more than a decade operating in relative secrecy, the department is gradually lifting the veil on its methods, allowing the FT unprecedented access to a handful of Prevent staff working in London and Kent.