The contact is believed to be UK Health Secretary Sajid Javid who tested positive on Saturday, though the NHS system does not confirm the name of a positive contact when it directs self-isolation.
The move means both Johnson and Sunak will be in self-isolation on so-called “Freedom Day” on Monday, when all of England’s legal lockdown restrictions come to an end.
Downing Street confirmed the decision by the Prime Minister and the Chancellor to self-isolate as a U-turn from an initial decision that Johnson and Sunak will both be participating in a daily contact testing pilot scheme to allow them to carry on essential government business from their workplaces. However, amid widespread criticism of such a plan, the Downing Street statement was updated hours later to align with the wider rules on self-isolation.
“The Prime Minister has been contacted by NHS Test and Trace to say he is a contact of someone with COVID,” a Downing Street spokesperson said.
“He was at Chequers when contacted by Test and Trace and will remain there to isolate. He will not be taking part in the testing pilot. He will continue to conduct meetings with ministers remotely,” the spokesperson said.
“The Chancellor has also been contacted and will also isolate as required and will not be taking part in the pilot,” the spokesperson added.
The move follows Health Secretary Javid’s COVID positive test on Saturday, when he revealed on Twitter that he was going into self-isolation with mild symptoms. It was expected that other Cabinet ministers he had been meeting with over the past few days may get affected.
An earlier Downing Street statement on Sunday morning read: “The Prime Minister and Chancellor have been contacted by NHS Test and Trace as contacts of someone who has tested positive for Covid.
“They will be participating in the daily contact testing pilot to allow them to continue to work from Downing Street. They will be conducting only essential government business during this period.”Javid tested positive on Saturday morning after a meeting at Downing Street the day before, and soon both Johnson and Sunak were contacted by NHS Test and Trace.
Downing Street initially said a workplace pilot scheme would allow them to keep working from their offices but Opposition parties expressed their fury, saying it suggested there was "one rule for them and another for the rest of us".
Sunak later tweeted: "Whilst the test and trace pilot is fairly restrictive, allowing only essential government business, I recognise that even the sense that the rules aren''t the same for everyone is wrong.
"To that end I''ll be self-isolating as normal and not be taking part in the pilot," the Indian-origin chancellor wrote.
Opposition Labour Leader Keir Starmer branded the Conservative Party government as being in “chaos”.
“Boris Johnson and Rishi Sunak have been busted yet again for thinking the rules that we are all following don''t apply to them,” said Starmer.
"The public have done so much to stick to the rules. At a time when we need to maintain confidence in self-isolation, parents, workers and businesses will be wondering what on earth is going on in Downing Street. The way the prime minister conducts himself creates chaos, makes for bad government and has deadly consequences for the British public," he said.
Under the rules in place, there is a legal obligation to self-isolate if the NHS Test and Trace directly calls someone to inform someone about a COVID positive contact. The NHS COVID contact tracing app also pings people to alert them of such a contact, but being pinged is only a guidance to self-isolate and not a legal requirement.
Latest figures show over 500,000 people were pinged by the NHS app in a week, leading to concerns over the system being over-sensitive and resulting in many deleting it from their smartphones to avoid self-isolation.
As daily coronavirus case numbers are continuing to rise across the UK ahead of the lifting of legal rules on social contact in England on Monday, businesses have raised concerns about staff shortages due to the numbers of people being told to self-isolate. However, the government has said its successful vaccination programme will serve as a wall between a rise in cases being followed by a rise in hospitalisations and deaths.