Motorists driving on smart motorways were forced to obey 50mph speed limits after a “secret” technology failure meant cars that broke down in live lanes could not be detected.

A whistleblower has leaked a classified memo which urged National Highways staff to be “vigilant” after a critical life-saving system failed in the West Midlands during the Easter holidays.

The email shows that an investigation was launched and a “major incident” declared as bosses admitted “the extent of the problem is not known”.

It adds: “The worrying thing is that presently the IT community do not know why this happened.”

This latest failing has again led road safety campaigners opposed to smart motorways – where the hard shoulder is turned into a fourth lane – to call on the Government to scrap the controversial schemes that have been linked to numerous fatalities.

Coroners have warned that more deaths could occur because replacing the hard shoulder with so-called “emergency refuge areas” and relying on technology such as “stopped vehicle detection” (SVD) radar is not foolproof.

‘Extent of the problem is not known’

The Telegraph has established that on April 17, the SVD radar covering much of the West Midlands suffered “reporting irregularities”.

On April 24, staff in control centres saw an “increase in the number of potential missed detections”, a National Highways source said, adding how there were “concerns about whether the [radar] technology was detecting stopped vehicles appropriately.”

As a result, gantries across both the West and East Midlands imposed lower speed limits, often 50mph, with roadside messages advising how the computer system, equivalent to the motorway network’s air traffic control, was “experiencing an issue”.