Labour criticised Mr Johnson for being absent from Cobra emergency meetings as the UK prepared for the first ever red extreme heat warning.
Downing Street insisted it was “not unusual” for Cabinet ministers to lead such meetings and that Mr Johnson’s Typhoon flight was “important” to understand the RAF’s capabilities.
Labour leader Sir Keir Starmer accused the Government of failing to focus on the heatwave, as the Tories fight a bitter leadership contest.
“For many people it’s going to be a real struggle … in the heat, and they’re seeing a Prime Minister who’s basically checked out, so he’s not really doing anything,” he told reporters during a visit to a central London bank.
“You’ve got a Cabinet that is completely distracted with the circus of a leadership race.”
Cabinet Office minister Kit Malthouse chaired a Cobra meeting on Monday, having held them on Saturday and Thursday, and defended Mr Johnson’s response.
He told MPs: “It’s literally my job as Chancellor of the Duchy of Lancaster to chair Cobra, in particular where the civil contingencies secretariat is involved, and then to brief the Prime Minister accordingly.”
Mr Malthouse later added: “It feels like a political attempt to create some air of panic about the next 36 hours and indeed it seems a politically-motivated assault upon the Prime Minister, which is completely unfair.
“He has been in touch with our work to co-ordinate across all of the nations of the United Kingdom, and I’m sure he will continue to do so.”
Mr Johnson shot a selfie video from a Typhoon fighter jet on Thursday, which he described as having taken off from RAF Coningsby in Lincolnshire like a “vertical firecracker”.
The Prime Minister, who is remaining in No 10 until September, told an audience at the Farnborough International Airshow on Monday that he was then given the controls.
“So I pushed the joystick right over to the right and we did an aileron roll, and I pulled the joystick right back and we did a fantastic loop the loop, and then I did a more complicated thing called a barrel roll and I pushed the stick up and right a bit,” he said.
Mr Johnson said they then “started to pull a few Gs” before he apparently lost consciousness and entered a dream about wind farms “harvesting the drained prairies of the North Sea”.
Over the weekend Mr Johnson held a party for friends at his country retreat, Chequers, where they were reportedly treated to burgers and sparkling English wine.
London Mayor Sadiq Khan said Mr Johnson should resign immediately rather than enjoy a “joyride” on a fighter jet and “go on a jolly”.
“The Met Office, for the first time ever, have issued a level four red alert warning, the chief medical officer is advising everybody to be careful as a consequence of this extreme weather, the Prime Minister is hosting a lavish party at Chequers and obviously going on a joyride on a Typhoon plane,” the Labour politician said.
“This idea of a Prime Minister, who has been voted out by his party, having a jolly for six months is treating the British public with contempt – he should go now.”
The Prime Minister’s official spokesman argued Mr Johnson has a role in directing the RAF in worst-case scenarios, “so it is important he is aware of some of these capabilities they have”.
Asked if he needs first-hand experience to understand this, the spokesman said: “It’s important that he has a detailed understanding of the working capabilities of the RAF.”
The spokesman also insisted that no taxpayer money was used for the Chequers party and added: “It is not unusual in Cabinet Government for Cabinet ministers to chair these sorts of things.”
But shadow levelling up secretary Lisa Nandy said: “We think the Government ought to do a number of things – first is to turn up to work.”
The Labour MP told Sky News the Prime Minister has “clearly clocked off”, adding: “And so have many of his ministers in his Government.”