"I understand that the head of MI5, Andrew Parker, briefed Cabinet ministers today, such is the seriousness of what they believed they have uncovered".
Nonetheless, he credits police and the MI5 domestic intelligence service with stopping most attacks at a time when Britain faces an unprecedented level of extremist activity.
Earlier on Tuesday, May's spokesman had said that Britain has thwarted nine plots in the last 12 months.
Two men were arrested last week following a joint operation by MI5, the UK's counter-terrorism security service, and police.
Trump May Sometimes Bypass Chief of Staff to Give Aides Orders
But Kelly can only keep tabs on Trump during working hours, and the president has often gone rogue when left alone. In the past, Trump had an open-door policy that allowed virtually anyone to have an audience with the president.
The disclosure came just hours after the United Kingdom government revealed that a terrorist attack on a Manchester concert venue that left 22 people dead could have been averted if security officials had responded differently to intelligence about the suicide bomber in the months before the attack. An official report into the attack released on Tuesday found opportunities to thwart it had been missed.
Sky News quoted its correspondent as saying there was an "extreme Islamist suicide plot against Downing Street (a reference to the UK Prime Minister's residence)".
In 1991 the IRA launched a home-made mortar bomb attack on Number 10. The then prime minister, John Major, was inside but was not hurt.
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