Concerns have been raised that thousands of drivers will be stuck in gridlocked traffic this weekend during an unprecedented closure of the M25.

The five-mile stretch of the motorway between Junctions 10 and 11 in Surrey will be closed in both directions from 9pm on Friday until 6am on Monday while a bridge is demolished and a new gantry is installed.

This is the first planned daytime closure of the motorway – which encircles London – since it opened in 1986.

Motoring experts are warning that official estimates of congestion levels may be “optimistic”, while local councillors in areas where motorway traffic will be diverted are anticipating “gridlock”.

The M25 normally carries between 4,000 and 6,000 vehicles in each direction every hour from 10am until 9pm at weekends between Junctions 9 and 11.

This includes many airline passengers travelling to, from and between the UK’s two busiest airports, Heathrow and Gatwick.

National Highways said modelling carried out while it was developing its plans for this weekend indicated vehicles would face delays of up to five hours without mitigation measures, such as urging drivers to stay away.

The Government-owned company believes its awareness campaign will help reduce traffic levels by approximately 50%, meaning the length of time added to journeys will be limited to around an hour.

An 11.5-mile diversion route has been created to direct motorway traffic along A roads.

National Highways is urging drivers not to use their sat navs to attempt to find quicker alternative routes on minor roads due to fears this could cause major congestion.

Steve Gooding, director of motoring research charity the RAC Foundation, told the PA news agency: “We must hope National Highways has overstated its dire predictions of sat navs adding to the traffic chaos by taking drivers off the official diversion route, because the temptation to try to skip the queues will be intense, and the impact on actual journey times uncertain.

“Whilst the modelling suggests around an hour might be added to people’s travel time, that will feel optimistic to anyone used to the frustrations of driving round the M25 on days even without major construction works under way.”

St Peter’s Hospital, Chertsey urged people to “only visit if it is necessary” this weekend.

It said in a statement: “Please be aware of the diversions that will be in place and that there is likely to be considerable congestion in the local area.

“Please plan your journey allowing extra time as it is likely to take longer than usual.”

Some school sports fixtures scheduled for this weekend in Surrey have been postponed because of the M25 closure.

Councillors whose wards are on the diversion route expressed concerns about what will happen.

Malcolm Cressey, of Runnymede Borough Council, said: “I think it’s going to be a difficult period but we have to sort out those bridges.

“I think it’s going to be very disruptive.

“I would certainly urge anybody to try and avoid the area.

“It could end up with all sorts of gridlock in our area.”

Tahir Aziz, of Woking Borough Council, said: “We’ve never experienced something like this before.

“It will have a significant impact in this area.

“It will cause huge disruption and delays, and a lot of traffic jams.”

Four more daytime closures of the M25 will take place up to September.

The project, due to be completed in summer 2025, will increase the number of lanes at Junction 10, which is one of the UK’s busiest and most dangerous motorway junctions.

Mr Gooding said: “National Highways needs to learn from this weekend’s experience and work closely with neighbouring road managers to minimise the traffic impacts of the further planned closures.

“It must give as much certainty and notice as possible – months rather than weeks – to drivers and hauliers, many of whom will be making trips that can’t be put off to another day.”

Earlier this week, National Highways project lead Jonathan Wade said it was tough to forecast the amount of congestion that will be caused by the closure because “it’s so unprecedented”.

He added: “It’s very difficult to determine right now how effective all our traffic management will be.

“Please don’t travel if you can avoid it.”