The main is on East Street (A158) - just yards away from where it meets the A153 at one of the busiest junctions in the county.
The work means contractors have had to erect temporary four-way lights on East Street, North Street, the Bull Ring, and Jubilee Way.
That, in turn, has led to gridlock on adjoining roads, particularly in the town centre and Conging Street.
A motorist told the News this morning: “It took my nearly 30 minutes to get from Louth Road to Boston Road.
“The delays were a nightmare. I was late for an important appointment.
“I can understand the repairs have to be carried out, but surely there’s a better alternative.”
According to residents, there was extra traffic in Stanhope Road, Foundry Street and Queen Street as drivers tried to avoid the delays.
On two occasions last week, Horncastle Police issued warnings for drivers to either avoid the junction or allow extra time for journeys.
Local gas network company Cadent said engineers were working to repair the leak as quickly as possible.
The company thanked drivers for their patience but said it wasn’t sure when the work would be completed.