It had been assumed for more than 30 years that supports bearing the Clifton suspension bridge were solid stone.

The 12 cavities, located under the south tower, were discovered by a team of specialist climbers and cavers exploring the Bristol-based bridge supports.

An exploratory bore hole was sunk into the stone back in 1969, but it missed the chambers and they remained hidden for more than 30 years.

The biggest cave is about three double-decker buses high

Mike Rowland, visitor centre

The chambers are said to be impressive places with huge vaulted ceilings and four-metre stalactites, but only experts will be allowed to access them.

The public will have to be content with an exhibition of photographs of the find at the bridge's visitor centre.

Mike Rowland of the centre said: "It started in 2000 when we were doing paving work on the Leigh Woods side of the bridge.

"We did an X-ray and found a shaft underneath the surface.

The chambers were uncovered by cavers

"Through 2001 and earlier this year we realised there was much more underneath there, but didn't know quite what.

"When we sent a team of professional cavers down there they found these huge caves, the biggest of which is about three double-decker buses high."

The 137-year-old suspension bridge was designed by engineer Isambard Kingdom Brunel.

Brunel, who also designed the wide gauge Great Western Railway, did not live to see his design completed, however.

The engineer died in 1858, while the impressive bridge was not opened until 1864.