The A1M was closed during the rush hour on Thursday morning because of a major gas leak under the hard shoulder.
The southbound carriageway was shut between junctions six and four at Hatfield, in Hertfordshire.
A Transco spokesman said all three lanes of the carriageway should reopen by Thursday evening.
There were long delays on roads in the area and other routes, including the A14 and the M11, which had become congested as a result.
Safety priority
Transco who were at the scene said an eight-inch diameter gas main which runs below the hard shoulder on the A1M southbound had been leaking.
Transco spokesman John O'Grady told BBC News: "We strongly advise motorists to avoid the A1 southbound and also avoid the surrounding area due to congestion.
"We have no accurate information as to when the repair work will be completed but safety is our priority," he added.
Mr O'Grady apologised to road users affected by the congestion and said an inquiry would be carried out.