Speed cameras on the M62 are working when the illuminated signs are off, it has been confirmed.

The news came to light after a Twitter exchange between a motorist and West Yorkshire Police, amid a claim a camera had flashed at Hartshead Moor when the gantry system was turned off.

Previously it had been thought motorists would only be automatically fined if they broke speed limits when the digital signs were on.

But a police tweet said: “Scheme is managed by @HAnews_yorks. It indicates a vehicle has exceeded national speed limit which applies when signs are off.”

West Yorkshire Police refused to comment further but the Highways Agency has now confirmed the new £150m overhead gantry system between Huddersfield and Leeds CAN catch speeding motorists, even when it is not being used to slow the traffic.

Click on the gallery below to see the managed motorway works and speed camera gantries

Campaign group, The Alliance of British Drivers (ABD) described the news as “unacceptable” while celebrity traffic lawyer, Nick ‘Mr Loophole’ Freeman, dubbed the development “underhand”.

A spokeswoman for the Highways Agency said: “The speed enforcement system is capable of enforcing variable speeds including the national speed limit, including when the signs are blank.

“Enforcement of the speed limit is a matter for West Yorkshire Police, but drivers are reminded that they must follow motorway regulations and obey the speed limits at all times.”

ABD spokesman, Brian McDowall, said it had been their understanding that the signs had to be illuminated for motorists to be caught speeding.

“It’s the first time we’ve heard of this,” he said.

“It’s unacceptable because otherwise you have no forewarning.

“For everyone’s understanding, it should be clear that if the signs are on then the limit is enforced, and if they’re off it is not being enforced.”

Mr Freeman said he would defend anyone who got a ticket for speeding when the signs were off.

He said: “It’s the most underhand and devious way of trapping speeding motorists.

“The expectation is the cameras are only used to measure the reduced speed limits.

“If the police or the Highways Agency are going to start using these cameras to measure speed during normal road conditions it’s grossly unfair.

“It’s imperative that at all times they display what speed is being enforced.”

AA spokesman, Edmund King, said the situation needed to clearer.

He said: “If the cameras are working when the signs are off then motorists should be told.

“There should be a sign that says ‘cameras in use’.

“They should be transparent about it, it should be clear what their objective is.

“The objective should be to get drivers to slow down and not to prosecute thousands for speeding.”

More roadworks are to bring disruption to the M62 this week.

Engineers plan to close lanes on the motorway overnight as part of a £4.5m plan to replace the central reservation barrier.

Two lanes on the eastbound carriageway between junction 25 (Brighouse) and 26 will be closed tonight and tomorrow.

Two lanes on the same stretch will be closed on the westbound carriageway tomorrow and Thursday nights.

All work is between 8pm and 5am.

The work on the barrier will run until April 2014.