Ministers gathered at the Cabinet Office on Saturday (16 July) to prepare for temperatures hitting an unprecedented 40C, after meteorologists warned there could be a threat to life.
The Prime Minister was reportedly hosting a party for friends and family at his luxury country retreat Chequers.
So why did Mr Johnson miss the Cobra meeting on Saturday, and what have those involved said about it?
What happened at the Cobra meeting?
Ministers held an emergency meeting on Saturday (16 July) to prepare for the potentially life-threatening temperatures the UK will see this week.
This comes after the Met Office issued its first red warning for extreme heat, covering London to Manchester and York for today (Monday 18 July) and tomorrow (Tuesday 19 July).
Mr Johnson was not present at the meeting - he was instead reportedly in Buckinghamshire at his country house retreat Chequers, where he hosted a leaving party for friends.
Following his resignation on 7 July, the Prime Minister is set to leave office in September once a new Tory leader is chosen.
Allies have said the gathering at Chequers was a “private event” and that no taxpayers’ money was spent on it.
The event went ahead after Mr Johnson and his wife Carrie were forced to abandon an earlier plan to use Chequers for a delayed wedding reception following heavy criticism.
What has the reaction been?
The Labour Party has criticised Mr Johnson’s absence, claiming he has “clocked off” from his role as Prime Minister.
Angela Rayner, Labour’s deputy leader, said: “Boris Johnson has gone missing in action again.
“He’s back to his old trick of skipping important Cobra meetings.”
She is most likely referring to the Cobra meeting on the Covid-19 pandemic that Mr Johnson skipped at the start of his premiership.
Ms Rayner continued: “The public will have no confidence in this zombie Conservative government responding swiftly and decisively to this national emergency as the disgraced prime minister prepares to party while Britain boils.”
Shadow Levelling Up Secretary Lisa Nandy added to Ms Rayner’s comments, stating it was clear the Prime Minister - and many of his ministers - have “clocked off.”
She continued: “We think the Government ought to do a number of things: first is to turn up to work.”
Ms Nandy argued there should be a dedicated Cabinet Office minister to coordinate an emergency response in situations such as these.
What Downing Street said about the Cobra meeting
A Downing Street source said it was “not unusual” for other members of the Cabinet to lead Cobra meetings.
Chancellor of the Duchy of Lancaster Kit Malthouse, who chaired the meeting, claimed it was unfair to criticise Mr Johnson.
He insisted the Prime Minister had been fully briefed on the situation.
Mr Malthouse added: “It’s literally my job to chair Cobra. The Civil Contingencies Secretariat sits in my department.
“[Mr Johnson] appoints secretaries of state to do this kind of work and that’s what I’ve been doing.”