London: There were a record 304 terrorism arrests in the 12 months to March, the highest level since 9/11, as the police and security services tackle an unprecedented terror threat level.

Home Office statisticians said the increase was driven by a 66 per cent increase in arrests of people from “white” ethnic groups, with 113 white suspects arrested in the 12 months to March compared with 68 the previous year.

The Home Office figures published on Thursday show that the total of 304 arrests includes 12 people arrested in connection with the Westminster attack in March but excludes arrests in connection with the Manchester or London Bridge attacks.

The scale of the terror threat facing Britain has been laid bare, with security service figures showing police and MI5 are carrying out 500 investigations involving 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.

The figures come as the prime minister has promised to introduce a four-point counter-extremism plan in the Queen’s speech, including longer sentences for those convicted of minor offences that are terrorism-related.

The number of terrorism-related arrests is an 18 per cent increase compared with the 258 arrests made in the 12 months to March 2016 and is the highest number on record 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 per cent over the previous year.

The figures show that 108 of the 304 arrests led to a charge, with 91 of them 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 per cent increase over the previous year and the largest number.

They led to 68 convictions, which was also a record number in any 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 the 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 also fell by a further 31 per cent 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.