The UK record for the hottest day in June was broken for a third day in a row on Friday, with temperatures reaching 37.3C in Suffolk, the Met Office has said.

The 50-year-old record was beaten by more than a full degree, highlighting the intensity of the heatwave which has gripped the country this week.

It has resulted in travel disruption, hundreds of schools closing, and six NHS trusts declaring critical incidents after being overwhelmed with patients.

But the scorching heat is expected to ease over the weekend, and a yellow warning for thunderstorms has been issued for Scotland and parts of northern England.

The provisional hottest June temperature was recorded in Santon Downham on Friday.

It surpassed the high of 36.7C in Merryfield, Somerset, on Thursday, which followed 36.1C (97F) recorded in Gosport, Hampshire, the previous day.

Wales saw a peak of 35.1C in the village of Hawarden in Flintshire, and Scotland’s highest temperature of 29.2C was recorded in Eskdalemuir, Dumfries and Galloway.

It was cooler in Northern Ireland, where a high of 25.6C was recorded in Katesbridge, County Down.

All four nations have experienced their highest temperatures of the year so far this week, with sweltering conditions of more than 20C continuing through the night for many in England and Wales – in what forecasters refer to as “tropical nights”.

A red warning for extreme heat, which means there is a risk of serious health effects, was in place across London, the east and south-east of England until 21:00 BST on Friday.

There are also amber heat warnings in place until Saturday evening for south-west England.

Health services have come under strain in some parts of the country.

As well as the NHS dealing with an increase in emergency patients suffering heat-related illnesses, Dr Hilary Williams, clinical vice-president at the Royal College of Physicians, said MRI machines and scanners had been affected by the heat.

She told BBC Radio 4’s Today programme “basic ward infrastructure” had been unable to cope, adding: “The patients are far too hot, we’re hearing reports of elderly care wards way over 30C.”

The London Ambulance Service (LAS) has seen about a 50% increase in 999 calls during the heatwave, its head Jason Killens told the BBC, a trend he thought would be seen across the country.

Killens said LAS had cancelled all non-essential training and meetings to deploy more staff to the front line.

At least 600 schools in England either completely or partially closed because of the heat on Friday, after a week in which more than 2,000 schools in England and Wales had shut their doors. Exact numbers on school closures were not available.

In Leicestershire, the family of a teenage boy who became the latest to drown while swimming in open water during the hot weather paid tribute to him.

In a statement, they said 13-year-old Hayden Jones-Powell would be remembered for his “big smile”, as authorities reissued warnings about the hidden risks of getting into open water.

Travel disruption continued into Friday evening rush hour for some, with some delays and cancellations.

Avanti West Coast, Great Western Railway and Transport for London announced changes to services because of the heat.

There were closures at some of the UK’s most well-known tourist attractions.

London’s Tower Bridge, Cutty Sark and Royal Observatory closed to visitors, as did Marwell Zoo in Hampshire.

Elsewhere, firefighters contained a wildfire in Derbyshire that had affected about 200 hectares (990 acres) of land on Tintwistle Moor since Wednesday.

The heatwave – which has affected other parts of Europe – was driven by a “heat dome”, an area of high pressure that traps heat, settling over Western Europe.

The high humidity accompanying the heat exacerbated the potential for health impacts, the Met Office said.

Some areas have seen intense thunderstorms triggered by the heat, including in Edinburgh, where the airport said there had been significant disruption.

While it is hard to link climate change to individual extreme weather events, scientists say climate change is making heatwaves more frequent and intense.

Friday night will be warm again in south-east England, with lows of 20C expected, but cooler for most places further north and west.

Thunderstorms are expected overnight in northern Scotland – where a yellow warning is in place – in addition to showers in other parts of the country and in Northern Ireland.

Isolated thundery showers may also push across parts of eastern England during the early hours.

The heat is expected to continue across Saturday in south-east and eastern England, where temperatures could reach 32C and an amber warning has been issued.

It will be slightly cooler for most, however, with highs of 27C in northern and western parts of England and Wales, and 23C in Northern Ireland and Scotland.

On Saturday night, isolated thunderstorms are possible in south-east and eastern England.

By Sunday, it will feel cooler and fresher across the UK. Temperatures will range from 16C in northern Scotland to 27C in eastern England.

The cooler weather will continue into next week, with rain at times, especially in the north and west of the UK.