The Highways Agency worked with contractors throughout the night and the tunnel reopened in time for this morning’s rush hour.
The agency said: “The tunnel was closed at approximately 5.30pm following the discovery of the fault and is expected to remain closed into the night.”
The £371m tunnel was built by Balfour Beatty and stretches for 1.2miles under the Devil’s Punchbowl.
It was opened in April 2011.