Police believed they had successfully traced the weapons used in a 2014 attack on a Jewish museum in Belgium to an antiques store in Bilbao, where they found around 10,000 guns.

A judicial investigation, however, found that the guns were antiques, of which only nine – almost all of them from the 19th century – could still be used to fire a shot, The Times reports.

The antiques store, whose owners and associates spent three months in prison as a result of the police error, had meticulously catalogued every weapon in accordance with the law.

Its owners had failed to bring the guns – some of which had been loaned out for film productions such as Pan’s Labyrinth and Guernica – in line with one new regulation, but were not guilty of terrorism.