Highways authorities have urged motorists to steer clear of a southwestern section of the M25 from Friday night, when the second planned weekend closure of the London orbital motorway takes place.

Tens of thousands of vehicles will be diverted on to local roads when one of the most used stretches of the UK’s busiest motorway is closed in both directions between junctions 9 and 10 to allow for the replacement of a gantry and the construction of a gyratory bridge. The closure will be in place from 9pm Friday until 6am on Monday.

Most drivers will be forced on or off the motorway further south-east at junction 8 near Reigate to join a 19-mile official diversion route.

Managers at National Highways are concerned that drivers could ignore warnings to avoid the area after the first M25 weekend closure in March passed off smoothly, despite fears of unprecedented traffic jams or “carmageddon”.

They insisted there was no room for complacency during the second planned closure, which will shut a longer stretch of motorway at a time of year when more cars are usually on the road.

The M25 reopened ahead of schedule after the last closure, when a five-mile stretch was shut to demolish a bridge. However, the more complicated nature of the engineering work this weekend – building rather than demolishing a bridge – makes an early finish to the disruption unlikely, National Highways said.

Jonathan Wade, a senior project manager, said: “We’re lifting 72 concrete beams into position and it’s a lot more precise and intricate. The works will take longer.”

But Wade said there was some contingency in the timings, adding: “The idea that you can’t use the M25 at 6am on Monday morning is not acceptable.”

He said that, during the first weekend closure, the number of cars passing junction 10 dropped from a normal rate of 146,000 every 24 hours to just 44,000 needing to be diverted, allowing traffic to move freely along the diversion routes.

This weekend’s closure – the second of five planned outages – affects a different, adjacent stretch of motorway, with longer diversion routes and the complication that while most vehicles will exit at junction 8, the tallest lorries will proceed another eight miles along the M25 to a diversion from junction 9.

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Wade said avoiding jams depended on the “robustness of the signage and messaging” and said he was concerned that motorists would not heed the advice. “The worst possible scenario is that they said it was going to be bad last time, let’s just ignore National Highways and do our own thing - it could flip on its head, Wade said.

He also warned motorists not to be tempted to stay on and follow the larger vehicles until junction 9, even if thinking “why would you leave a perfectly good motorway?”, as it would lead them to a longer, slower diversion. Those following their satnavs could also find themselves gridlocked or liable to Ulez charges, if in non-compliant cars, as the diversions enter the Greater London clean air zone.

An exception to the main diversion will also made for be guests booked in to the Ramada hotel at Cobham Services on the M25, who will get a “special escort” from junction 9. Although it could seem a location of questionable allure during the closure, Wade said: “You’ll certainly have a quiet night in.”

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