Train operators have warned passengers to avoid travelling early next week due to extreme heat, unless their journey is “absolutely necessary”.

The UK Health Security Agency has increased its heat health warning from level three to level four – a “national emergency” – and the impact of soaring temperatures will disrupt public transport.

Temperatures of around 35C in Wales and southern England on Tuesday could also cause up to 20% more motoring breakdowns a day compared with the average for July, according to the RAC.

The national message from train operators is that customers should avoid travelling on Monday and Tuesday if possible.

TransPennine Express (TPE) has also urged anyone taking essential journeys on these days to pack water and sun cream.

All operators have warned passengers to expect last-minute cancellations and delays from Sunday.

This is partly due to speed restrictions imposed by Network Rail on Monday and Tuesday in response to the heatwave.

Operations director at TPE Kathryn O’Brien said: “With weather warnings in place between Sunday and Monday and a number of speed restrictions planned to be imposed on Monday and Tuesday, we will be running an amended train plan and are urging people to only travel if absolutely necessary on these dates.

“This is likely to be some of the hottest weather we’ve ever seen and it’s really important that anyone making an essential journey is prepared and brings water and sunscreen along with them.

“Journeys will take longer than usual and further disruption is likely. Anyone using our services on Monday and Tuesday should check before they travel and allow plenty of extra time.”

Avanti West Coast has warned passengers who wish to use services between London Euston and Scotland to plan ahead due to the impact of the heat on rail lines.

An expected rise in drivers flocking to coastal locations to soak up the sunshine this weekend is also likely to lead to traffic queues, the RAC has said.

The AA has warned that cars could feel like “mobile microwaves” and urged motorists to prepare for hot journeys by packing snacks, medication and water, and ensure pets and children are not left in passenger seats.

The car insurance and breakdown company has advised motorists to set off for work early, when temperatures are cooler, to reduce the chances of their engine overheating, which is the most common cause of breakdowns in hot weather.

Drivers should also check their vehicle’s cooling system with a mechanic, particularly owners of older cars, according to the AA.

The Met Office has issued a red warning for Monday and Tuesday, covering an area from London up to Manchester and the Vale of York.

Tuesday is likely to be the hottest day, with the potential to top the UK record temperature of 38.7C (101.7F) set in Cambridge in 2019.