Unusual traffic jams on the main north-bound road network in the past few days were caused by works on a section of the Santa Venera tunnel where significant amounts of water have been seeping out for months.

The north-bound carriageway was closed while government workers installed a protective sheath at the base of the leaking roof to channel seeping water to culverts on the side of the road.

The government denied the works were being done because of the Pope's visit, insisting the Papal entourage would not pass through the tunnel.

A spokesman for the Environment Upgrade Committee of the Office of the Prime Minister said the source of leaking water had not yet been found, although it was established the liquid was not sewage.

He said architects had recommended a temporary solution to prevent the water from gushing onto the road to the detriment of motorists.

The spokesman said investigations into the source of the water were made difficult by the presence of buildings, which prevented the digging of exploratory wells above the tunnel.