Thousands flocked to beaches amid warnings high temperatures over the next few days could result in people dying.
Climate attribution scientist at the Met Office, Dr Nikos Christidis, said the 40C prediction is a result of climate change.
The forecaster issued an amber warning for the majority of England on Sunday. The alert extends to southern Scotland and Wales from Monday until Tuesday.
Ms Nicholls told Sky News’s Sophy Ridge On Sunday programme: “This isn’t like a lovely hot day where we can put a bit of sunscreen on, go out and enjoy a swim and a meal outside.
“Obviously there is some common-sense practical advice we are talking about – stay hydrated, stay out of the sun at the hottest times, wear sun cream – those sorts of things,” he told Sophy Ridge On Sunday.
Ministers held a virtual emergency Cobra meeting on Saturday after meteorologists warned the record high temperatures could put lives at risk.
Avanti West Coast, Transport for London and London North Eastern Railway are also urging passengers not to travel unless it is absolutely necessary.
Meanwhile, the chairman of the NHS Confederation said hospitals are going to be “really, really pushed” over the next few days.
Greater Manchester Police implored people to avoid cooling off in reservoirs, rivers or ponds after a teenage boy died while swimming with friends in a canal.
