The Wiltshire and Swindon Safety Camera Partnership (WSSCP) claims that in three months, traffic topping the 70mph limit had dropped from 55% to 36.28%.

"This is a direct result of enforcing the limit, nothing else has been done to slow traffic," a spokesman said.

But opponents have criticised the figures as "outrageous spin".

Paul Smith, from the Safe Speed road safety campaign, said: "We need accurate statistics to allocate resources to help save lives."

In April several hundred motorists travelled in two convoys at about 56mph across a 30-mile stretch of the motorway in Wiltshire protesting about the cameras.