Boris Johnson will miss a third COBRA meeting on the heatwave as Labour said the Prime Minister had "clocked off".
A meeting of the Government's emergency committee will be held at 2.30pm, chaired by Chancellor of the Duchy of Lancaster Kit Malthouse.
Downing Street defended the PM's absence from the third crisis briefing in a row after he spent the morning at Farnborough Air Show, in Hampshire.
The PM's official spokesman was unable to say what he would be doing when the COBRA meeting would take place.
He is expected to face MPs in the Commons later this afternoon as MPs stage a vote of no confidence in the Government.
Asked what Mr Johnson would be doing when COBRA was happening, his official spokesman said: “I don’t have his diary in front of me so I don’t know if there is a clash.
“I will say more broadly on this and I think Kit Malthouse himself made the point that he is taking the lead on the Government’s response to the heatwave.
"He is keeping the Prime Minister fully briefed, including over the weekend, where the PM also spoke to a number of secretaries of state about the work that they are doing.
“It is not unusual in a Cabinet government for cabinet ministers to chair these sorts of things.”
It comes after Mr Johnson did not attend a COBRA meeting on Saturday and instead worked from his Chequers retreat - ahead of a party hosted with his wife Carrie on Sunday.
Another COBRA meeting was held on Friday.
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."
She told Sky News the Prime Minister has "clearly clocked off", adding: "And so have many of his ministers in his Government."
Mr Johnson gave a jokey speech to open the Farnborough Air Show in Hampshire today, where he cracked gags about sending MPs into outer space.
In a thinly veiled dig at his mutinous Cabinet, he said: "I leave it to you to imagine who at this stage I would like to send into orbit.
"Perhaps a volunteer can be found from the green benches of Parliament.
"I leave that entirely to you, into your speculation.
"But for now, with so much to look forward to in this incredible sector, and with the UK at the leading edge of progress, not just for our national security and the security of our friends and neighbours, not just for our economic prosperity around the whole country, but for the protection of the planet itself, I declare this great Farnborough Air Show open."
Downing Street defended Boris Johnson for not chairing Cobra meetings on the heatwave - while finding time to take a ride in a Typhoon fighter jet last Thursday.
Asked about Mr Johnson's Typhoon flight, the spokesman said: "The Typhoon's quick alert reaction role is an integral part of both UK and Nato security and the PM does have a specific role in directing RAF action 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."
But Mayor of London Sadiq Khan said Boris Johnson is treating the British public with "contempt”.
Mr Khan said: “If the Prime Minister wants to go on a jolly, he should leave Number 10 Downing Street, he should resign and allow the Deputy Prime Minister to be a caretaker prime minister until the Conservatives have chosen their leader."
He added: "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."
