Locations of some of the secret new speed cameras on the M25 in Kent have been revealed.

The Highways Agency had refused to confirm the sites because it could "compromise the effectiveness of the scheme and road safety" as part of a £129million upgrade.

But it can now be revealed that at least two have been fitted around a mile either side of Clacket Lane in both directions.

Dozens of Highways Agency Digital Enforcement Camera Systems (Hadecs) have been installed around the M25.

Each site is understood to be able to contain up to six cameras, depending on the number of lanes. The cameras are grey and mounted on grey poles, rather than being bright yellow.

Improvements on a 12-mile section of road, which spans between J5 for Sevenoaks and J7 for the M23, finished at the end of April and the road now features speed cameras for the first time.

Campaign group the Alliance of British Drivers said the scheme is potty.

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."

Dubbed a "smart motorway" by the Highways Agency, the stretch of road contains 38 CCTV cameras, 88 overhead signals, 33 signs on verges, 13 emergency telephones, 10 refuse areas and nine gantries that span both carriageways.

It is claimed that smart motorways bring extra capacity quicker and cheaper than traditional road widening schemes, while remaining equally as safe.

"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..." - Hugh Bladon

It does this by using the hard shoulder as a permanent running lane to reduce congestion and ease traffic flow.

A Highways Agency spokesman said they could not give out specific information on camera locations because it could compromise the effectiveness of the scheme and road safety.

The spokesman added: “All that is happening is that we are introducing new, more advanced cameras which do that better.

“We found that on the M42 near Birmingham, where a similar scheme exists, compliance with speed limits is very, very high indeed.

“That’s not particularly to do with speed cameras being there, it’s a stretch of motorway that people use regularly.

“They understand if the sign reads 60 and they stick to it they get to there in a nice, smooth traffic flow.

“People who use it regularly realise speeding up and speeding down for cameras isn’t particularly productive.

“Regardless of cameras, signed speed limits are enforceable.

“It is important that drivers understand that the onus is on them to ensure that they abide by speed limits.”

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