The 1.2 mile tunnel, which at its deepest point lies 65 metres underneath the Surrey Hills Area of Natural Beauty, was expected to open in August 2011.

However, thanks to the work of about 600 construction staff, the project is seven weeks ahead of schedule and there is hope it will open next July.

Passengers were taken from one end of the tunnel to the other by the Highways Agency for the first time on Monday.

Roads Minister Mike Penning was among those being chauffeured through. He said: "It was absolutely humbling to be the first person from the non-construction side to drive through it."

The next major milestone for the project will be when the 1.5 mile section of the southern approach road opens to traffic next month.

Construction works are expected to be finished by the end of the year, before experts begin six months of safety tests inside the tunnel to check fire risks, water supplies, speed cameras and electrics.