Record-breaking temperatures could hit parts of Britain for the rest of the week as the school holidays get under way in much of the country.

Temperatures will increase as the week progresses, according to forecasters, from a high of 33C (91.4F) on Tuesday to 34C on Wednesday and rising up to 37C before the end of the week, which would be a July record. If temperatures reach at least 25C for three consecutive days in the majority of the UK, it would be classified as a heatwave.

The Met Office tweeted that temperatures reached 31C at Heathrow airport at around midday. Although the main concentration of the hot weather will be in central and south-eastern England, it could reach the high 20s in parts of Scotland, where most schools broke up earlier this month.

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The humid conditions are expected to lead to thundery showers across south-west England on Tuesday evening and the early hours of Wednesday morning, which could cause travel delays and power cuts. Scattered patches of lightning, hail and winds are predicted to move north from Tuesday evening, according to the Met Office.

A yellow severe weather warning covering most of England, Wales, and Scotland began at 6pm on Tuesday and will run until 9am on Wednesday. Public Health England has also renewed warnings about the heat, urging people to keep hydrated, find shade and take protection against the sun.

Marco Petagna, a Met Office forecaster, said there was the potential for record-breaking temperatures by Thursday evening. He said there was a 60% chance that Thursday’s weather would surpass the July record of 36.7C set at Heathrow in 2015 and a 30% chance of beating the all-time UK temperature record of 38.5C (101.3F), recorded in Faversham, Kent, in August 2003.

Petagna said: “On Thursday, we’ll see an east and west split with showers in the western parts of England but the eastern parts will look very hot with 36 to 37C (96.8F).

He said the coolest areas would be in western England where temperatures will remain in the 20s, adding that anywhere across England could see some thundery showers.

Ruth May, the chief nursing officer for England, urged people to check on their neighbours during the warm weather. She said people should take care as temperatures rise, while Asthma UK urged sufferers to keep up their medication.

May said: “The NHS will be there always for anyone who needs it, but everyone can help by checking in on vulnerable friends and neighbours, while making use of the free, convenient and helpful phone and online NHS services for minor illnesses, to help frontline staff provide care for those in emergency and serious need.”

A dog welfare charity has advised owners to not leave pets alone in a hot car even for a few minutes. Dogs Trust says on days when the temperature is 22C it could rise by 11C in 10 minutes inside a car and dogs could not cool down the same way as humans.

Earlier this week, an American bulldog received treatment after his body temperature soared to a life-threatening 42.2C in Glasgow.

Paula Boyden, Dogs Trust’s veterinary director, said: “There are so many things we can do to make sure our dogs stay happy and healthy in hot weather, but it is crucial we keep a close eye on them, even if we are playing indoors. If we all do this, then hopefully we and our dogs will be able to enjoy a long hot summer.”