A total of nine Islamist terrorist plots have been thwarted in the UK over the past year, the director general of MI5 has told Cabinet.
Andrew Parker told Theresa May and her senior ministers that the defeat of Islamic State in Iraq and Syria did not mean the terror threat was over, warning that social media was being used to encourage attacks in the UK and elsewhere.
Home Secretary Amber Rudd told the weekly meeting in 10 Downing Street that the pace of attack planning has increased "significantly" over the past year.
Mrs May told ministers that the threat from terror was at an "unprecedented" level, despite the military setbacks experienced in its Middle East strongholds by Islamic State - also known as Daesh.
In a regular Westminster media briefing, the Prime Minister's official spokesman declined to discuss the details of the attacks that had been prevented over the past year, some of which are subject to forthcoming court proceedings.
But he confirmed that the subject of Mr Parker's presentation to Cabinet was the threat from Islamist-related terror.
A senior counter-terror police officer said earlier this year that 13 attacks had been foiled by security services since 2013.
But the pace of plotting is understood to have been stepped up in a year which has seen deadly Islamist-inspired attacks in Manchester, Westminster and London Bridge, as well as the bombing of a Tube train at Parsons Green and a van driven into pedestrians outside a mosque in Finsbury Park.
"Mr Parker said that nine terrorist attacks have been prevented in the past year," said Mrs May's spokesman. "There have been five attacks that have got through, four of which are related to Islamist terrorism.
"The Prime Minister gave thanks to the tireless work of staff at MI5 to combat the unprecedented terrorist threat.
"Ministers heard that, while Daesh had suffered major defeats in Iraq and Syria, that doesn't mean that the threat is over. Rather it is spreading to new areas, including trying to encourage attacks on the UK and elsewhere via propaganda on social media.
"The Home Secretary said the pace of attack planning had increased significantly this year.
"Amber Rudd said that the Government has been putting pressure on social media companies to remove terrorist material and progress was being made.
"She pointed to recent efforts by Facebook, which last week said 83% of Islamic State and al Qaida content was being identified and removed within one hour.
"The Home Secretary stressed there was more for social media companies to do."