LONDON (Reuters) - The number of people arrested in Britain on suspicion of terrorism offences rocketed by 68 percent in the last year to the highest figure on record during a period when the country suffered four deadly attacks, figures showed on Thursday.

Statistics from the Home Office (interior ministry) showed there were 379 arrests in the year to June, up from 226 from the 12 previous months, and the most since 2001 when the data began to be collected.

Britain is on its second-highest threat level, “severe”, meaning an attack is highly likely and 36 people were killed in terrorist incidents in the first six months of 2017.

Among the arrests, 12 came after an attack in March on London’s Westminster Bridge when a man drove a car into pedestrians killing four, before he stabbed a policeman to death outside parliament.

Another 23 followed a suicide bombing at a pop concert in Manchester in May, and the following month police arrested 21 suspects after three Islamist militants drove into pedestrians on London Bridge before stabbing people at nearby restaurants and bars, killing eight.

One arrest followed an attack in north London when a van was driven into worshippers near a mosque which left one man dead.

Britain’s most senior counter-terrorism officer Mark Rowley has said police have been arresting a suspect every day. He said this week that there had been a shift in the threat level rather than an isolated spike.

In the three years until March this year, police foiled 13 potential attacks but in the next 17 weeks, there were the four attacks while the authorities thwarted six others, Rowley said.

“The pace has continued to be almost as challenging since then,” he told a conference in Israel.

The official figures showed that among the 379 arrests, 123 people were charged with an offense, of which 105 were terrorism-related, while 189 were released without charge.