Nine terrorist attacks have been prevented in the UK in the past year, two more than had been made public, the Cabinet has been told.
MI5 Director General Andrew Parker revealed the figures in a presentation at Downing Street on Tuesday on the current terrorist threat facing Britain.
Parker told the Prime Minister that the defeat of the so-called Islamic State in Syria did not mean the terrorist threat was over and warned that social media was being utilised to incite attacks from afar.
MI5 has previously said that 13 terror attacks were prevented in the past four years, a third of which were assisted by public information.
In October it was revealed that counter-terror investigators are estimated to be running 500 live investigations involving 3,000 individuals at any one time.
Parker released the information ahead of the publication of an investigation into the security services and police following a string of terror attacks in Britain last year.
David Anderson QC has examined the role of the security services in four terrorist attacks in the UK to assess what intelligence was held on the perpetrators ahead of the terror strikes and what could be done better.
The former independent reviewer of terror legislation is said to have submitted his classified report to the Home Secretary in October.
Anderson reviewed the attacks at Westminster Bridge on 22 March, the bombing of Manchester Arena on 22 May, the London Bridge attack on 3 June and the incident at Finsbury Park Mosque on 19 June.
The attack at Parsons Green on 15 September was not covered by the report.