Boris Johnson is set to miss a crisis meeting on the response to the heatwave because he is hosting a farewell party at his country estate, according to reports.
The Met Office put out an extreme heat warning amid fears soaring temperatures could spark health problems and even deaths.
Experts have warned there will likely be a risk to life as large swathes of the country swelter under highs of 41C on Monday and Tuesday.
A third COBRA meeting on the issue, chaired by the Duchy of Lancaster Kit Malthouse, is expected to take place over the weekend.
But Boris Johnson will be at Chequers and is reportedly unable to attend the meeting, Sky News reported.
Mr Johnson is due to host a party on Sunday to thank loyal MPs, aides and No10 staff before a new Prime Minister is declared on September 5.
Angela Rayner, Labour’s deputy leader, condemned Mr Johnson for being “missing in action”.
Ms Rayner said: "Boris Johnson has gone missing in action again. He’s back to his old tricks of skipping important COBRA meetings.
"Where’s the plan for the delivery of essential services and how people will be kept safe at work, on transport, in schools, hospitals and care homes?
"The public will have no confidence in this zombie Conservative government responding swiftly and decisively to this national emergency as this disgraced prime minister prepares to party while Britain boils.
"If he still can’t take the responsibility of the job, he should leave right now."
No10 has been contacted for comment.