Drivers have been warned to steer well clear, turn off the satnav and stick to the official diversions as some of the biggest roadworks since the Romans results in part of the M25 motorway fully closed for the first time in its history this weekend.

Epic traffic jams are anticipated, with a five-mile stretch of Britain’s busiest motorway, the London orbital motorway, closed for engineering from 9pm on Friday.

This weekend marks the first ever planned daytime closure of the M25, in what could prove a marathon ordeal for motorists in south-west London and Surrey.

The M25 will be closed in both directions at what is normally one of its busiest points, between junctions 10 and 11, until 6am on Monday, while a bridge is demolished and a new gantry is installed.

Despite months of painstaking planning, National Highways is uncertain how long the tailbacks will be but said modelling suggested that without mitigation measures – mainly telling drivers to stay away – that queues could hit five hours.

About 4,000-6,000 vehicles an hour normally travel on that stretch of the M25 and will now be diverted on smaller roads for more than double the length, about 11.5 miles.

The stretch of motorway is used by airline passengers travelling to, from and between the UK’s two busiest airports, Heathrow and Gatwick.

Drivers who do need to find their way on and off the M25 have been warned to ignore the siren call of the satnav and find their own back roads, with fears of greater gridlock should vehicles stray from the official diversion route.

Cars are expected to clog the zigzag diversion via Cobham and Woking, up the A3 and down the A245.

National Highways hopes that the warnings will keep traffic to about half of normal levels and add only about an hour to journey times.

The agency’s project lead, Jonathan Wade, said: “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.”

Steve Gooding, the director of the pro-motoring thinktank the RAC Foundation, told the PA news agency: “We must hope National Highways has overstated its dire predictions of satnavs 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.

“While 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.”

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.

Councillors 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.”

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