3,000 suspects, but just 60 watched full-time

MI5 can monitor only one in 50 of the 3,000 jihadis considered worthy of investigation, it was revealed last night.

Restricted resources mean the security services can monitor only 0 terrorist suspects around the clock. A surveillance operation requires 30 officers per suspect, plus manpower to monitor electronic communications.

It came as the Home Office revealed the number of terror arrests in Britain has surged to a record high after a wave of deadly attacks.

Some 400 suspected extremists were held in counter-terror swoops in the year to September. The rise of 54 per cent from 259 in the previous year came as the UK threat from jihadis and far- Right extremists reaches unseen levels. There were five lethal terror attacks in the UK between March and September, killing 3 , for which a total of 4 suspects were detained.

Earlier this week, Home Secretary Amber Rudd disclosed that Britain’s police and spies had foiled nine plots since the Westminster Bridge attack in March – and thwarted 22 since the murder of soldier Lee Rigby in May 2013.

But restricted resources mean only a fraction of the hundreds of suspected Islamist extremists at large can be subject to intensive full-time surveillance at any one time. The Home Office data revealed that 12 suspects were held after the attack at Westminster Bridge, 23 in connection with the bombing at Manchester Arena, 21 over the London Bridge atrocity, one arrested after the Finsbury Park mosque attack and seven linked to the Parsons Green Tube blast.

Some 58 of the 400 arrests were of suspected female extremists, the highest since records began in 2001. Eighteen were under 18, more than any other year. The number of white terror suspects held is at a new high of 143.

The statistics also show that the number of people in prison for terrorism-related offences reached 213 – an increase of 44 on the previous year.