"I understand that the head of MI5, Andrew Parker, briefed Cabinet ministers today, such is the seriousness of what they believed they have uncovered.
"It is in essence an extreme Islamist suicide plot against Downing Street. Essentially police believe that the plan was to launch some sort of improvised explosive device at Downing Street and in the ensuing chaos attack and kill Theresa May, the Prime Minister.
"This is something which has been pursued over several weeks at least by Scotland Yard, MI5 and West Midlands Police.
"It came to a head last week with the arrest of two men, by armed police, who were charged with preparing acts of terrorism."
Naa'imur Zakariyah Rahman, 20, from north London and Mohammed Aqib Imran, 21, from south-east Birmingham are due to appear at Westminster Magistrates' Court on terror charges on Wednesday morning.
On Tuesday MI5 revealed that it had prevented nine terror attacks in the UK in the past year but several attackers have still got through.
In March, Khalid Masood drove a car at pedestrians on Westminster Bridge and stabbed a policeman guarding Parliament. Five people died in the attack.
In May, 22 people died when suicide bomber Salman Ramadan Abed targeted concert-goers at Manchester Arena. An official report into the attack released on Tuesday found opportunities to thwart it had been missed.
Eleven people died when terrorists used vehicles and knives to attack people at London Bridge in June.
A man died in a terrorist incident at Finsbury Park mosque in north London in the same month - and, in September, an attempt to bomb a Tube train at Parsons Green station failed.
Prime ministers have long been terrorist targets, though Downing Street is now well protected by fortified gates and armed police officers.
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 launched a home-made mortar bomb attack on Number 10. The device landed in the back garden. The then prime minister, John Major, was inside but was not hurt.
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