There were a record 304 terrorism arrests in the 12 months to March, the highest number since 9/11, as the police and security services tackled an unprecedented threat level.
Home Office statisticians said the increase was driven by a 66% rise in the number of white suspects arrested, up to 113 in the 12 months to March compared with 68 the previous year.
The Home Office figures published on Thursday include12 people arrested in connection with the Westminster attack in March but exclude people detained in connection with the Manchester and London Bridge attacks.
Security service figures show the police and MI5 are carrying out 500 investigations involving a total of 3,000 suspects at any one time. There are also 20,000 former “subjects of interest” who they say they need to keep under review.
Theresa May has promised to introduce a four-point counter-extremism plan in the Queen’s speech, including longer sentences for people convicted of minor offences that are terrorism-related.
The number of terrorism-related arrests was up 18% from the 12 months to March 2016 and was the highest number for any financial year since data collection started in September 2001.
The quarterly statistics also show that as of 31 March there were 186 people in prison for terrorism-related offences and domestic extremism – an increase of 15% on the previous year.
The figures show that 108 of the 304 arrests led to a charge, with 91 suspects being charged with terrorism-related offences. One hundred were released without charge and the remainder bailed pending further action. So far, 33 of the 91 arrested on terrorism-related offences have been prosecuted and 31 found guilty. A further 53 were awaiting prosecution.
Seventy-nine terror trials were completed in the 12 months to March – a 55% increase over the previous year and the largest number on record. They led to 68 convictions, which was also a record number for a financial year.
The statistics show that those convicted for terrorist offences are getting increasingly longer sentences. Six were given life sentences, four are serving 10 years or more, and 28 received sentences of between four and 10 years.
While arrests are at a record high, police use of stop and search, cordons and examinations at airports has fallen to record lows. Only 453 street stop and searches were carried out by the Metropolitan police on anti-terror grounds.
The number of people held at airports and ports under schedule 7 of the Terrorism Act 2000 fell by 31% to 18,103, and 25 police cordons were set up in Britain in the 12 months to March – 13 fewer than the previous financial year.
European security chiefs said on Thursday that the squeeze on Islamic State in Iraq and Syria was increasing the risk of stay-at-home jihadis.
The warning was contained in the annual terrorism report of the European policing agency, Europol. The report lists a total of 142 failed, foiled and completed terrorists attacks in eight European states in 2016, more than half of them in the UK.
The Europol list for the UK is at odds with the figures released by the UK security services, which has reported between 2013 and the end of 2016 only a handful of attacks and 13 foiled plots.