Drivers are being warned of massive delays when the M25 shuts in both directions this weekend in an unprecedented move.

The closure between junctions 10 and 11 in Surrey on the UK’s busiest motorway could cause chaos.

When exactly is it going to be shut, where are the diversion routes and why is it happening? Here’s everything you need to know.

When is the closure and how long will it last?

It’s from 9pm on Friday 15 March to 6am on Monday 18 March and covers the five-mile stretch between junction 10 and 11.

What is the diversion route?

Here’s the diversion route that’s been outlined by National Highways, which maintains England’s motorways:

• Junction 10 to junction 11: Northbound A3 to Painshill Junction, A245 towards Woking, and then A320 to M25 junction 11
• Junction 11 to junction 10: A320 south towards Woking, A245 towards Byfleet and Painshill junction, Southbound A3 to junction 10.

You can see it on the map below:

And here are some Google Maps screenshots showing roads that are part of the diversion route:

Satnav warnings

Drivers are being urged to ignore satnavs and only follow official diversion routes to prevent causing gridlock during an “unprecedented” closure.

Jonathan Wade, National Highways project lead, said the amount of disruption will partly depend on whether drivers stick to official diversions.

“How many people are going to take the initiative and try and use satnavs?”, he told PA news agency.

“There’s probably a greater risk of congestion by people just doing their own thing and thinking they can perhaps beat the signs and find a shorter or quicker route.

“That will cause further congestion on some of the key junctions so please avoid doing that if at all possible.”

National Highways senior project manager Daniel Kittredge said: “If people move away from diversion routes that we prescribe, it creates additional issues in different parts of the road network.

“The majority of the time that will be local roads, so that really impacts residents in those particular areas.

“That’s why we’re trying to encourage people to not follow the satnav.

“Stick on the prescribed diversion route. It’s going to be more suitable for your journey.”

How bad could it be?

It’s the first scheduled daytime all-lanes shutdown on the M25 since it opened, so it’s not yet known exactly how bad delays are going to be.

This section of the M25 normally carries between 4,000 and 6,000 vehicles in each direction per hour from 10am until 9pm at weekends, so the disruption caused by the works is expected to be significant.

More than 200,000 vehicles are expected to be affected, including many travelling in and out of London, and to and from Heathrow and Gatwick airports and Channel ports.

What advice has been issued?

“Drivers should only use the M25 if their journey is absolutely necessary,” says Jonathan Wade, National Highways project lead.

“This is the first of five full closures of one of the busiest junctions on our road network,” he adds.

“We have spent months planning for these closures and making sure there are diversion routes in place, but there will still be heavy congestion and delays.”

‘Motorists should decorate the bathroom’

The government-owned company’s chief also advises motorists to find something to do at home like “decorate the bathroom” or “play in the garden” ahead of the closure.

Mr Wade says how well the area copes with the closure will partly depend on whether drivers stick to official diversions – but urges people to avoid travelling altogether.

“Avoid the area totally if you can,” he told The Independent’s daily travel podcast.

“Either avoid travelling completely or find something to do at home, decorate the bathroom or something, I don’t know, or play in the garden.

“If you must go, travel by train, walk, use your bicycle.

“If you can, avoid driving anywhere around those diversionary routes.”

Airport warnings

People due to travel to Gatwick and Heathrow could also be affected by the closure.

Heathrow Airport is advising passengers planning to use this part of the M25 to allow for extra time before their flight.

“Passengers using public transport should also be aware that The Airline (between Heathrow and Gatwick) and RailAir (RA2), will be running amended timetables over this weekend, please check with your operator for the latest information,” their statement adds.

A London Gatwick Airport spokesperson told Sky News: “Passengers driving to the airport are advised to check diversion routes before they travel and allow extra time for potential delays.

“Gatwick’s train station is well connected and is a great alternative option for people travelling to the airport this weekend.”

‘You ain’t seen nothing yet’

Steve Gooding, director of motoring research charity the RAC Foundation, says: “For drivers who’ve already had their patience tried by the queues at the junction 10 works, the phrase ‘you ain’t seen nothing yet’ springs to mind.”

“National Highways’ plea for people to avoid driving in the area applies not just to trips on the M25, but also to those on surrounding local roads onto which the M25 traffic will be diverted,” he adds.

“The hope must be that drivers take great care, however frustrating the delays and disruption might be.

“The last thing we need is shunts or crashes, however minor, because the slightest mishap will compound the misery.”

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Why is it happening?

Government-owned company National Highways said the action is necessary to enable a bridge to be demolished and a new gantry to be installed as part of a £317m improvement project.

National Highways says the project 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.

“These improvements will bring long-term benefits to drivers who pass through this stretch of the M25, not to mention pedestrians, cyclists and horse riders who will also see positive changes in the area,” says its project lead Jonathan Wade.

Is the closure a one-off?

No – it’s just one of five planned full closures between the junctions. The other dates have not yet been confirmed.

“Three of those closures will be between junctions 10 and 11 – the A320 at Chertsey… and two of them will be between junction 9 at Leatherhead to junction 10 at Wisley,” Mr Wade said.

He said the dates of the later stages would be released in due course with motorists given plenty of notice.

“We will not just spring them on people,” he said, adding they would take place between May and December.

The project began in summer 2022 and is expected to last three years in total.