New 'smart' speed cameras on the M25 have already caught 700 drivers in just two months – and they could be coming soon to a motorway near you.

The devices, which use digital technology for the first time, have been introduced on one section of London's orbital motorway between junctions five and six in Kent to catch motorists breaking the 70mph speed limit.

And the new gantry models, which are painted grey to make them harder to spot than the standard yellow, seem to be working – with 668 drivers receiving fines since they were introduced last year.

Of those, 148 penalties related to drivers exceeding 40 to 60mph variable speed limits, but the majority, 520, were for breaking the 70mph limit, The Times reported.

More of the devices, which have been dubbed 'stealth cameras' by some critics, were installed on a northern stretch of the M25 before Christmas, and are set to be rolled out to parts of the M1, M3, M6 and M60.

It means that thousands of motorists could be at risk of paying out at least £100 in fines and getting points on their licence if they stray over the limit.

It is part of a plan by the Highways Agency to expand 'smart' motorways and so ease the flow of traffic. Other steps include using the hard shoulder as an extra lane at peak periods.

RAC data finds 800 more people would be killed or seriously injured each year without cameras (Getty Images)

But the new cameras – which use front and rear-facing cameras to verify speed, and can scan four lanes at a time – have been criticised by motoring organisations, who say the move is designed to raise income through fines and is not about road safety.

They also claim that drivers will be penalised for driving only marginally over the speed limit on stretches of the motorway that have previously been unpoliced.

Professor Stephen Glaister, director of the RAC Foundation, told The Times: "The law is the law but it needs to be applied consistently. Many constabularies have followed the ACPO guidance.

"If the new approach is one of zero tolerance then it needs to be equally applied across the network and understood by motorists and police forces."

Studies have shown that as many as 95 per cent of drivers admit to breaking the limit on motorways.

The government had planned to raise the motorway speed limit to 80mph before the proposals were shelved by Transport Secretary Patrick McLoughlin, in 2013.

The Association of Chief Police Officers recommends that drivers are not charged unless they exceed 79mph in a 70mph zone.

A Highways Agency spokesperson 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.