Britain's domestic intelligence chief says the country is facing the worst terrorist threat he's seen his 34-year career.

MI5 Director General Andrew Parker said in a rare public speech on Tuesday that the threat is "multi-dimensional, evolving rapidly and operating at a scale and pace we've not seen before."

Parker says there has been a "dramatic upshift" this year, which has produced attacks in London and the city of Manchester that have killed 36 people combined.

Parker says another 20 attacks were foiled in Britain over the past four years and "many more" were prevented.

He cited a record number of terrorism-related arrests, including 379 during the first six months of 2017.

Parker says it's impossible for MI5 to stop every attack, but that the service is identifying and disrupting threats all the time.