Within a 2km stretch of the river 6,000 fish were found dead, including 1,500 salmon, 650 trout and 3,200 bullheads.

The Mint in Llantrisant said it "very much regrets" the incident and has agreed to pay to restock the river.

The Environment Agency Wales served the enforcement notice on the plant on Monday.

It said that the level of pollution would normally lead to prosecution but that the Royal Mint was protected from such action by Crown Immunity.