A seven-mile stretch of the M25 is set to shut in both directions from tonight for the second of five planned weekend closures.
Motorists have been advised to avoid the area of the closure - between junctions 9 and 10 in Surrey - from 9pm on Friday to 6am on Monday morning.
Drivers have also been urged to stick to the planned 19-mile diversion route, which will send vehicles through parts of Surrey and Greater London.
It is the second of five planned closures of the motorway, and comes after a section between junctions 10 and 11 was shut in March.
On that occasion, fears diversion routes would become overwhelmed proved unfounded, as traffic levels were more than two-thirds lower than normal after a widespread awareness campaign.
However, National Highways senior project manager Jonathan Wade says there is a "very real" risk that the lack of disruption in March means some motorists will not heed warnings to replan journeys this weekend.
"We've upped the amount of correspondence that we've been sending out to try and counter that, but it remains an issue," he says.
Those set to be those travelling to, from and between the UK's two busiest airports, Heathrow and Gatwick, are expected to be affected as National Highways has warned it expects long delays.
Other stretches of the M25 will also be disrupted this weekend due to ongoing work to retrofit additional emergency stopping areas on smart motorway sections.
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What are the diversion routes?
Diversion route clockwise for all but over-height vehicles:Leave M25 at junction 8, A217 (Reigate). Follow the A217 London, Sutton, (A240) Kingston. After 3½ miles turn left onto the A240 Epsom, Kingston. After 3 miles at the Esso roundabout, turn right onto the A24 (A240) Kingston. Continue for 3 miles and turn left onto the A3 Portsmouth, Guildford. Continue for 9½ miles to the M25 and re-join the motorway at junction 10.
Diversion route anticlockwise for all but over-height vehicles:Leave M25 at junction 10 to join the A3 towards London. After 9½ miles at the Hook interchange leave the A3 turning right onto the A240 Epsom, Reigate. Continue for 3 miles to the Esso roundabout and turn left onto the A240 Reigate. Continue for 3 miles then turn right onto the A217 Reigate, M25. After 3½ miles turn left to re-join the M25 motorway at junction 8.
Meanwhile, Mr Wade warned drivers will be subjected to London's ultra low emission zone (ULEZ) rules if they move off official diversion routes during this weekend's M25 closure.
National Highways said anyone ignoring 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.
Diversion routes will take cars on a 19-mile journey on A roads, crossing from Surrey into London's ULEZ area.
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National Highways senior project manager Jonathan Wade told the PA news agency: "Although the (ULEZ) cameras will be active, no enforcement action will be taken.
"However, if you ignore the diversion signs and do your own thing, then if your vehicle's not compliant you do run the risk of getting caught.
"You'll be perfectly safe as long as you follow the diversion routes."
Read more from Sky News:
Everything you need to know ahead of the M25 closing
Aerial pictures show calm amid M25 gridlock fears
RAC spokesperson Alice Simpson also warned drivers not to rely on their sat navs during the closure.
"Planning ahead is absolutely vital," she said.
"Rather than relying on a sat nav, check the planned diversion routes ahead of time and be prepared for long delays.
"It's also a really good idea to check your vehicle's oil and coolant levels, tyre pressure and tread depth all before setting off to reduce the chances of a very unwelcome breakdown."
National Highways say this weekend's closure is to allow for the installation of a new bridge between junction 9 and junction 10.
Be the first to get Breaking News Install the Sky News app for freeThe previous closure was to allow for the demolition of Clearmount bridleway bridge between junction 10 and junction 11 and the installation of a large gantry.
Three more weekend closures of the M25 are set to take place between August and the end of the year.
The project, due to be completed in summer 2025, will increase the number of lanes and make it easier to enter and exit the M25 at Junction 10, which is one of the UK's busiest and most dangerous motorway junctions.
