Officers raided the building on Saturday night and found what they believe to be a "sophisticated" operation producing guns, of which the NCA has subsequently seized around 30.

Three men found leaving the building were arrested and police had to use a Taser on one of them, an NCA spokesman said.

On the weekend, two guns were found as well as ammunition, but further searches uncovered the true magnitude of the factory.

Rob Hickinbottom, head of the national firearms threat centre at the NCA, said on Wednesday afternoon: "We believe we have disrupted a group involved in the criminal production of firearms, and as a result we have prevented a potentially large quantity of weapons from getting into the hands of criminals and being used in violence on our streets."