The UK's hottest day in May for more than 50 years has kicked off the bank holiday weekend, but forecasters are warning of unsettled weather ahead.
Londoners have sweltered as the temperature hit 31C
London temperatures reached 31C (88F), the hottest May day since 1953, causing rail problems for three Tube trains.
The RAC said that at its worst, motorists on the M25 suffered a 90-mile traffic jam clockwise from junction 26.
The Met Office said rain and haze would affect the south on Monday, with temperatures only in the mid-teens.
The RAC motoring group said there had been "various accidents and the sheer volume" of traffic had led to bottlenecks on the M25, which built up from mid-afternoon as the hot weather prompted many to head off on holidays.
The M4 and the M5 were also busy both ways around Bristol during Friday afternoon, it said.
The A394 in Devon and Cornwall was partly closed due to an accident and there were extensive tailbacks.
But in south east England, roads were "not much busier than you'd expect for a sunny Friday", the spokesman said.
The AA said an estimated 18 million cars would be on the roads and it was increasing its patrol numbers by a fifth.
"Drivers may get caught out if they do no preparation for their journey, especially if they get stuck in queuing traffic, as this is when cars, like us, are more likely to overheat," spokesman Gavin Hill-Smith said.
"Five minutes spent checking things like your car's coolant and oil levels, battery, tyre tread and pressures really can help prevent problems from arising."
The temperature of 31C was recorded at the BBC's London Weather Centre on Friday afternoon.
A BBC weather spokeswoman said: "The last time that we had a temperature over 30C in May was in 1953, when it reached 31.7C at Heathrow and Farnham."
In a tunnel south of Baker Street station, three Tube trains were stuck for an hour after points failure due to the heat, a Transport for London spokesman said.
Although five vehicles from London Ambulance Service were sent to the scene, only two people needed treatment.
The bank holiday comes as the Good Beach Guide released figures suggesting the number of UK beaches failing European water quality tests has doubled.
Airports operator BAA said officials were expecting a steady weekend, with more than 1.78m passengers due to pass in and out of its seven airports.
Top destinations included New York for Heathrow passengers, and Spanish resorts such as Malaga, Palma de Mallorca and Gerona from Gatwick and Stansted.
The Association of British Travel Agents (Abta) said more than 2.1m people were expected to travel abroad over the weekend, saying Spanish resorts were the most popular.
But other destinations, including Turkey, Egypt and Florida, were also proving attractive.
"The summer season gets into full swing this weekend and we are looking forward to a bumper number of bookings," said Abta president Martin Wellings.