Motorists in Essex have been warned of lengthy delays as parts of the M25 close over the weekend, which could see the hottest day of the year so far.

The motorway – which is Britain’s busiest – will be closed in both directions between junctions 9 and 10 in Surrey from 9pm on Friday until 6am on Monday while concrete beams for a new bridge are installed.

Saturday is predicted to see temperatures rise as high as 26C in parts of south-east England, including around London, in what would be the hottest day of the year.

Diversion routes will take cars on a 19-mile journey on A roads, crossing from Surrey into London’s Ulez area, though National Highways has assured drivers “no enforcement action will be taken”.

But the highways authority said anyone who ignores official diversion signs in an attempt to find shorter alternative routes will be liable for the £12.50 daily Ulez fee if their vehicle does not meet minimum emissions standards.

Failure to pay the charge when required can result in a £180 fine, reduced to £90 if paid within 14 days.

The RAC advised motorists to avoid travelling if possible and pack extra food and drink if they do, as the closures are likely to bring long delays for thousands of drivers.

Alice Simpson, RAC breakdown spokesperson, said: “As this next M25 weekend closure immediately follows major national rail strikes which could see many more people take to the roads, it could be up to six days of disrupted travel for London’s drivers.

“The closures also coincide with the retrofitting of new emergency areas on all-lane-running stretches of the M25, meaning delays are likely between junctions 5 and 7 in Surrey and Kent and junctions 23-27 in Hertfordshire and Essex.

“With weekend temperatures reaching 25 degrees or even higher, it’s likely that routes around the 23-mile diversion will be busy as people make the most of the fine weather and jump in the car for day trips.

“Anyone heading to Chessington World of Adventures, which is close to the official A3 diversion route, should expect delays and pack extra food and drink in the event they’re on the road longer than expected.

“Holidaymakers heading to Gatwick or Heathrow should leave an hour earlier than usual and allow plenty of time for parking and security checks.”

The M25 closure is the second of five planned weekend closures which come as part of a £317 million project to improve Junction 10.