Cadent has discovered gas is escaping in small amounts from a pipe about one metre below Rookery Road, near Wolverhampton and Coseley.
Engineers say although the escape – at the junction with Birmingham New Road – is small in volume and well below levels to require action such as evacuation, it has to be fixed before it gets any worse.
On Tuesday, the team moved in under a one-way closure – but after a series of 'near misses' with motorists and pedestrians ignoring warning signs, a full closure is now in place.
The repair is expected to take a few days, but no date for the reopening of the road has been confirmed by Cadent.
Craig Hansbury, customer area operations manager for the Black Country, said: “We know this isn’t ideal, but hope people understand it’s essential work and a full closure is needed to keep everyone safe as we complete the work.
“At this stage, we anticipate the repair will take a couple of days to complete and then we will get the road reinstated to good order and ready to reopen.
"Please follow the diversion signs in the meantime. We really appreciate everyone’s patience.”
There should be no impact to local gas supplies.