Drivers have been issued an urgent warning to ensure they do not get caught out with a ULEZ fine when the M25 closes on Friday.
The motorway will close from 9pm on Friday, May 10 to 6am on Monday, May 13 between junctions nine and 10 and drivers have been warned to stick to the official diversion or face a fine.
National Highways has told drivers of non-Ulez-compliant vehicles entering the clean air zone they will not be charged as long as they stay on the marked route, but will otherwise be liable for the £12.50 charge if they follow their satnav for an attempted shortcut.
Only petrol vehicles more than 18 years old or diesel cars which are more than nine years old are liable for the charge implemented by Mayor of London Sadiq Khan, which was expanded to cover the whole of Greater London last year.
The diversion to rejoin the motorway runs for 19 miles along A-roads in Surrey and Greater London.
Jonathan Wade, a National Highways senior project manager, told the PA news agency: “Although the cameras will be active, no enforcement action will be taken.
“However, if you ignore the diversion signs and do your own thing, then if your vehicle’s not compliant, you do run the risk of getting caught.”
The traffic agency has urged all drivers to follow the main route to minimise the risk of rural gridlock, with thousands of cars normally using the stretch of motorway every hour.
This weekend is the second of five planned closures of the M25 in a £317m works scheme, after the first ever planned daytime shutdown in March on the neighbouring stretch from junctions 10 to 11.
The closure of the busy south-western stretch has brought fears of long delays and congestion for thousands of drivers.
Work will see concrete beams for a new bridge and a new gantry lifted into place.
