UK thwarted 13 terror plots, probing 500 more

British security agencies have launched a new campaign to gather information about possible terror attacks from the public after foiling 13 potential attacks in nearly four years.

Britain’s security services are dealing with 500 live counter-terrorism investigations after foiling 13 potential terror attacks since June 2013, including some of the scale of the Paris and Brussels attacks, according to a top counter-terrorism official.

File photo of Metropolitan Police assistant commissioner Mark Rowley speaking to the media outside New Scotland Yard in central London in January 2014. (Reuters)

Mark Rowley, assistant commissioner of Scotland Yard, revealed the figures on Monday as he launched an appeal called “Action Counters Terrorism” for the public to report suspicions. He did not disclose details of the 13 foiled plots.

Describing the public's contribution to foiling plots as "extraordinary", Rowley told BBC: "Some of that information is a change in someone's behaviour, some of that's about suspicious activity. Sometimes that public information has actually started an investigation.”

He added investigators have been making arrests at a rate of close to one a day since 2014, as the official threat level for international terrorism has stood at “severe” - meaning an attack is highly likely - for more than two years.

Rowley said the range of threats and challenges included encrypted communication methods, propaganda and various possible attack methods. "Now we worry about everything from fairly simple attacks with knives or using vehicles all the way through to the more complex firearms attacks," he said.