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The fourth arch of Bridgend’s medieval old stone bridge has been uncovered for the first time in hundreds of years.

The structure, parts of which could be almost 600 years old, was found behind a wall in an office building’s basement after rainwater leaked in.

A number of bones were also discovered, sparking a call to police.

But they have now been identified as animal, probably a cat, and possibly dating from the 18th century.

John Squire, director of Graham Paul chartered accountants in Dunraven Place, said they knew the arch was in their basement, but they had never seen it in full until builders pulled down a water-damaged wall.

“It’s very exciting. There’s a lot of interest in it,” he said.

He said after builders began work on repairs, some of the infill in the archway collapsed, exposing bones.

But South Wales Police subsequently identified the bones as animal, not human.

Originally built around 1425 to help pilgrims on their way to St David’s in West Wales cross the River Ogmore, the old stone bridge was severely damaged by flooding in the 18th century and was rebuilt in 1775, after which only two arches were visible in the span crossing the river.

But according to Welsh historic monuments body Cadw, which is responsible for the bridge as a scheduled ancient monument, the hidden structure had been described by John Leland (c. 1538-1540), who referred to the structure as “Bridges on Ogar Pennebont of four arches of stone”.

The Ordnance Survey also described two arches in the ground floor of a building in 1957.

The third arch is currently visible in the rear yard of the office building, which was constructed around 1830.

Both Cadw and the Glamorgan-Gwent Archeological Trust have inspected the newly-revealed arch, with experts from the Trust identifying the animal bones as coming from a cat, which may have got trapped under the arch during repair work in the 18th century.

A spokesman for the organisation, which has produced a report on the archway for Cadw, said: “To see some of the parts of the bridge that are hidden away in the basement is a rare opportunity.”

Its report, which details the damage suffered by the structure and what repairs are needed, has now been passed onto Cadw.

A Cadw spokesman said: “The Bridgend Old Bridge is a medieval bridge designated as a scheduled ancient monument and a Grade II listed building.

“The bridge is recorded in a deed of 1452. Cadw inspectors have visited the site and are providing advice to ensure that appropriate materials and techniques are employed to preserve the medieval fabric of the bridge and to assist the owner in complying with any applications required for listed building or scheduled monument consent.”