Wartime secrets buried below a North-East town centre are threatening a £20m redevelopment.

In 1939, Middlesbrough Council ordered the installation of seven, 27ft long air raid shelters to protect 350 town hall staff in the event of a German raid.

It is known that the Home Guard sometimes used such underground bunkers to store munitions.

The shelters were installed below Victoria Gardens, in the town centre, at a depth to allow the park and its flowers beds to be restored on top.

Now the council plans a £20m showpiece art gallery and civic square on the site of the gardens, but first must locate the shelters to be sure of the ground for foundations.

Council engineer Dave Screeton said: "If they still remain, then this could be a potential hazard.

"We have to carry out investigations to determine whether they are still there and if so, their condition."

A month ago, the council made an appeal to former employees to help them locate the shelters. But the plea fell on deaf ears.

However, late in the first day of the excavation yesterday, a council team found a vintage, underground toilet and then the entrance to a shelter, with tunnels running off it. The shelters were connected underground to the Town Hall.