Drivers using two sections of the M25, in Kent, Surrey, Essex and Hertfordshire, are being warned to keep to temporary speed restrictions, despite no obvious work being carried out.

National Highways says the speed limits, between junctions five and seven and junctions 23 and 27, are so newly installed technology can be tested and fine tuned.

The equipment will detect vehicles that have stopped on the motorway.

CCTV and free recovery is in operation while the speed limits are in place.

Nick Wells from National Highways said: "These measures are in place as an extra safety precaution until testing is complete and the system is meeting our rigorous performance specification.

"We are grateful for drivers' patience during this time and are working hard to restore the national speed limit at the earliest possible opportunity."

National Highways said the testing process usually takes about 12 weeks, although no specific end date has been set.

It said the testing can only happen when there are no roadworks and when drivers are able to use all lanes of the motorway.