The lifespan of Manod Quarry site in Llan Ffestiniog, famously used to hide priceless works of art during the Second World War, has been extended until 2048.

A planning application to Gwynedd Council said there was “sufficient reserves of slate remaining in the quarry to maintain production” until then.

The quarry, now known as Cwt y Bugail Quarry, is operated by Breedon Group subsidiary Welsh Slate.

Applicant Shaun Denny told Monday, 24 June’s planning meeting there was still “strong demand” for Welsh slate tiles, and accepting the application would “allow the quarry to increase expenditure”, thus “increasing the number of slates that could be sold” and helping to provide more “skilled jobs for the area”.

He said it would also help the nationally important cultural and heritage site by protecting its “history and place in the community” and allow time to record the remains of the gallery used as a storage facility for art during the Second World War.

Committee members expressed their support for the traditional Welsh industry and followed officers’ recommendations to approve the application with conditions.

Planners described how in 1940 the government built “a specialist and secret storage facility” in the tunnels and caverns below Manod to keep works of art, predominantly from The National Gallery in London, safe. The vast caverns created by the slate quarrying industry provided strong protection against aerial attack.

Manod was chosen due to its proximity to the railway and its remoteness, only accessible by a winding mountain road.

Underground works made the mine suitable for storage with brick built humidity controlled chambers and a narrow gauge rail track to move artwork around.

By the summer of 1941 the National Gallery collection was safely stored in the quarry. It remained there for four years.

According to Cotswold Archaeology’s report, “paintings began to arrive from August 12, 1941, with completion of the transportation of 2,200 pictures by September, 1941”.

The success of Manod’s repository is shown in requests from private collectors and institutions to store their art. It was also used by the Royal Family to safely store the royal collection, with drawings by Leonardo, Holbein, Claude and Michelangelo sent to Manod.

During the Cold War the Ministry of Works retained a lease on Manod Quarry and continued to “update and maintain the existing below-ground landscape”.

Underground work continued “on a limited scale” in parts of the mine not controlled by the Ministry until around 1965.

Site works resumed in 1982 with the reopening of Graig Ddu Quarry; reports stated that in 1983, the Ministry of Works relinquished its use of the storage facility.

Despite continued quarrying, the storage facility was retained. However planning documents note “the stability of the Manod Quarry workings had deteriorated considerably over the past 20 years”.

It added: “Today, the wartime storage facility is in a deteriorating condition predominantly due to the lack of ventilation and the instability of the quarry roof which has resulted in major rock falls.”

Experts still considered its presence to be of “high heritage value”, however.