The head of Britain's domestic spy agency on Tuesday accused Russian military intelligence of being on a "sustained mission" to cause mayhem in Britain and Europe.

In a rare public speech from MI5's counter-terrorism operations center in London, MI5 Director General Ken McCallum said ongoing efforts by autocratic states to harm Britain's security represents "the most complex threat environment we have ever seen."

McCallum said a total of 43 late-stage attack plots had been foiled by MI5 and police since 2017, noting that some plotters were seeking to get hold of firearms and explosives "in the final days of planning mass murder."

The spy chief said the agency was having to deal with a "dizzying range" of beliefs and ideologies, citing risks posed by Russia, Iran, China and others.

McCallum said the number of state threat investigations MI5 is running had jumped 48% in the last year.

"The U.K.'s leading role in supporting Ukraine means we loom large in the fevered imagination of [Russian President Vladimir] Putin's regime, and we should expect to see continued acts of aggression here at home," McCallum said.

He said the GRU — Russian military intelligence — in particular was on "a sustained mission to generate mayhem on British and European streets: we've seen arson, sabotage and more."

A spokesperson for Russia's foreign ministry was not immediately available to comment when contacted by CNBC on Tuesday.

McCallum said more than 750 Russian diplomats have been expelled from Europe since Moscow launched its full-scale invasion of Ukraine in early 2022, "the great majority of them spies."

"This goes well beyond all historical precedent – and has put a big dent in the Russian intelligence services' ability to cause damage in the West," he added.