Ministers are preparing for a surge in demand on the NHS and other essential services ahead of a heatwave that could result in record-breaking temperatures in Britain.

The Met Office has issued an amber extreme heat warning across much of England and Wales from Sunday to Tuesday, with temperatures expected to exceed 35C at their peak in southern, central and eastern areas of England.

After chairing a meeting of the Cobra emergency civil contingencies committee in Whitehall, Kit Malthouse, the Cabinet Office minister, admitted that health services could come under increased pressure and warned that temperatures could hit 40C.

“The key thing we can do is prepare the government services for what may be a surge in demand – not least the health service and elsewhere – but also, critically, communicate that the first line of defence is actually individual behavioural change,” he told BBC Radio 4’s The World at One.

“People need to take care, do all the stuff they would do when it is very hot – wear a hat, drink water – but critically also the most vulnerable groups – the elderly, those with cardiovascular problems and the very young – that people look out for them and take care.”

He urged members of the public to be alert to vulnerable people, to check on neighbours and to take precautions.

“We are asking people if they have an elderly neighbour, particularly if they live alone, it would be a neighbourly thing to check up on them and make sure they are looking after themselves and have access to water,” he said.

“The current forecast is that we will be in the mid to high-30s, but there is a smaller possibility that we could exceed that and possibly hit 40C, which would be an all-time record.”

Age UK said the extreme heat could be particularly challenging for older people with existing health issues, especially those affecting their heart or lungs.

No 10 said there were “tried and tested” plans in the NHS in place for increasing staffing and that there were contingency measures either in place or ready to be enacted. “There have already been some coordination meetings at official level across government,” Boris Johnson’s spokesperson said.

“This is a progression of that to ensure that, as we come into what are predicted to be the hottest days, that the necessary mitigations and actions are in place or being put in place.

“A lot of this is making sure the public are aware of the advice that is available – particularly those with young children or vulnerable family members.”

It came as new figures showed that staff absences at NHS hospitals in England were at their highest level for nearly three months amid rising Covid infections.

An average of 22,918 hospital staff in England were off work each day in the week to 6 July – a rise of 30% on the previous week – either because they were self-isolating or sick with Covid-19.