In a speech delivered less than 24 hours after the Prime Minister and the Chancellor performed a U-turn on not self-isolating, the Labour leader said the lifting of remaining coronavirus restrictions on so-called Freedom Day was “reckless”.
Sir Keir said the UK Government had “got virtually every big decision wrong either in substance, or timing, or both”.
And he called for the release of a list of Conservative ministers who had been part of a trial where they would not have to self-isolate after coming into contact with someone who had tested positive for coronavirus.
It comes after Mr Johnson and Mr Sunak were “pinged” by the app after coming into contact with health secretary Sajid Javid, who subsequently tested positive for the virus.
The pair initially tried to get around the requirement to quarantine by saying they would join a daily workplace testing programme being trialled by the Cabinet Office and No.10.
However, they were forced into a hasty reversal on Sunday amid widespread public anger.
Sir Keir said: “With family events cancelled, businesses having to close and workers having to go without pay, Johnson and Sunak’s attempts to dodge isolation were crass and insensitive.
"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.
"That’s why I’ve written to the Prime Minister, because there are still far too many unanswered questions.
"The Prime Minister claims he ‘did look briefly at the idea’ of taking part in the pilot scheme.
“This rides a coach and horses through the claim that Boris Johnson and Rishi Sunak had been magically selected for this trial.
“And it is completely inconsistent with a Downing Street press release saying they were ‘participating’ in the pilot.
“Let’s be clear, Boris Johnson and Rishi Sunak only went into isolation because they were busted.
“But we have seen their instincts on show yet again: it’s one rule for them, and another for the rest of us.”
Sir Keir also questioned whether the Prime Minister had travelled to his country estate at Chequers after learning Mr Javid had symptoms.
"Will the government now release a full list of Conservative ministers who have benefitted from the trial and avoided self-isolation?” he said.
"Many businesses and individuals have struggled to do the right thing and follow the rules. They would have benefited from this policy.”
The vast majority of Covid restrictions have been removed in England as of Monday under what has been dubbed as ‘Freedom Day’, with nightclubs resuming and the wearing of face masks in many public spaces no longer mandatory.
Scotland instead moved to level zero restrictions on Monday, with nightclubs still not allowed to reopen and mask wearing mandatory in spaces such as supermarkets.
Asked whether he was comfortable with nightclubs reopening and social distancing being dropped, Sir Keir said: “The big difference between us is that we think lifting all these restrictions, all these protections, in one go today is reckless.”