The tunnel from Luqa to Kirkop has been closed because of urgent road works, Transport Malta and the police said.
A big traffic jam built up, and motorists were urged to avoid the area.
Times of Malta reported last week that concerns had been raised about shoddy road surfacing in the tunnel.
On Friday, Transport Malta said upgrading of Triq l-Ewwel Titjira 1915, including the Kirkop tunnels, were to be ready by Saturday.