Two miles of congestion was seen on the M25 in Surrey on Saturday morning after a section of it was closed.
A five-mile stretch of the motorway between junctions 10 and 11 was closed in both directions at 9pm on Friday and will remain inaccessible until 6am on Monday while a bridge is demolished and a new gantry installed.
Concerns have been raised that thousands of drivers will be stuck in gridlocked traffic over the weekend during the unprecedented closure, with one councillor calling it a “nightmare”.
National Highways South East (NHSE), in a post to X, formerly Twitter, said there were two miles of tailbacks on approach to the closure at junction 10 on Saturday morning.
Meanwhile, traffic began to build along the main diversion route through Byfleet, West Byfleet, Woking and Ottershaw.
NHSE estimated the average journey time along the diversion route was 25 minutes, assuring drivers of minimal congestion.
It is the first planned daytime closure of the motorway – which encircles London – since it opened in 1986.
An 11.5-mile diversion route has been created to direct motorway traffic along A roads.
Daryl Jordan, of Woking Borough Council, told the PA news agency that residents will be “affected massively”.
He said: “It’s going to be a nightmare.”
South East Coast Ambulance Service, which covers Surrey, urged drivers to clear the way for ambulances ahead of a “challenging weekend” for crews.
Business owners in the areas along the diversion route have told of cuts they have made to their services in anticipation of traffic.
Mark Pollak, owner of Billy Tong, which caters for events and sells biltong at markets, told PA he expects to see 50% of the firm’s turnover for the weekend go “down the drain”.
Mr Pollak said he had to refuse a request for Billy Tong to cater an event in Guildford and had to cancel its stall at Surbiton Farmers’ Market on Saturday, with staff not wanting to face expected traffic to get to jobs.
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 nine and 11.
This includes many airline passengers travelling to, from and between the UK’s two busiest airports, Heathrow and Gatwick.
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.
By Ted Hennessey, Ellie Ng and Neil Lancefield, PA