Sky News called it an "extreme Islamist suicide plot", but offered no further details about the plan.
As per reports, police have arrested two men who were planning to blow up Downing Street gates and kill PM May.
Over several weeks, MI5, along with Scotland Yard and West Midlands Police, had been pursuing the case.
"It came to a head last week with the arrest of two men, by armed police, who were charged with preparing acts of terrorism".
MI5 investigators misinterpreted intelligence on Salman Abedi earlier this year and it was disclosed his case was due to be discussed at a meeting scheduled for nine days after his May attack at the Manchester Arena.
The suspects were detained during raids in London and Birmingham last week and charged with terrorism offenses. The same report found that the ringleader of the June London Bridge attack was actively under investigation at the time.
Security measures were introduced in the 1970s but were increased in the 1980s as the threat from Irish republican groups grew.
In 1991, the IRA managed to breach security, launching a homemade mortar bomb attack on Number 10. Analysts say the chances of the men gaining entrance to the prime minister's residence would have been remote.