The agency is devoting an increasing share of its activity to monitoring individuals who have received terrorist training or are plotting attacks, a parliamentary report disclosed.
It also flagged up serious concerns about extremists driven out of Iraq and Syria and laid bare the extent of Islamic State’s murderous ambitions, with an arm of the group said to be plotting terrorism in the West “pretty much all day every day”.
The disclosures are contained in the Intelligence and Security Committee’s annual report for 2016-17.
Members are given access to highly classified material and take evidence from cabinet ministers and senior intelligence officials.
Committee chairman Dominic Grieve said: “The scale of the terrorist threat facing the UK is unprecedented in terms of the number of current investigations and the overall number of ‘individuals of interest’.
“MI5 have told us that it represents a pace which they have not experienced before.”
MI5 described how “the most striking shift in the composition of CT (counter-terrorism) casework in the last five years was the proportion of what we refer to as high-risk casework”.