Hundreds of speeding drivers have been fined following the introduction of new 'secret' speed cameras on the M25.

The system of grey-painted cameras, installed on several sections of the M25 including 12-mile stretch between J5 for Sevenoaks and J7 for the M23, caught almost 700 drivers in just two months.

The Highways Agency Digital Enforcement Camera Systems, or Hadecs, are organised into sites containing up to six cameras, depending on the number of lanes.

Unlike traditional models, the Hadecs are grey and mounted on tall poles, making them much harder to spot than the familiar bright yellow casings of old-style cameras.

And there are other differences too.

Hadecs do not rely on marks on the ground to determine a vehicles speed, instead comparing two images taken by front and rear facing cameras.

The high-tech digital system is also capable of scanning not just one but four lanes simultaneously.

Installed in Spring last year, the sheme was branded 'potty' by campaign group the Alliance of British Drivers.

Speaking to KentOnline at the time, spokesman Hugh Bladon said: "They are trying to put in speed cameras wherever they can. They are doing it little by little in the hope no one will notice.

"The ultimate aim, I am sure, is to have speed cameras all over the motorway.

"We should concentrate on the way people are driving, rather than the fact they may be travelling over 70mph.

"We should spend money on more coppers picking up on things cameras can’t, such as driving under the influence of drink and drugs, lane crossing and tailgating."

Hadecs are part of what has been dubbed a "smart motorway" by the Highways Agency.

The 12-mile stretch of the M25 in Kent contains 38 CCTV cameras, 88 overhead signals, 33 signs on verges, 13 emergency telephones, 10 refuge areas and nine gantries that span both carriageways.

A Highways Agency spokesman said: “Variable speed limits on smart motorways are primarily there to smooth traffic flow, reduce congestion and make journeys more reliable.

“Hundreds of thousands of motorists use this stretch of the M25 every day. The vast majority are sticking to the speed limits and are experiencing better journeys as a result of smart motorways.

“There are clear signs where cameras are in place and the new cameras are more visible than the previous versions.”