The announcement comes at a time when the government is reviewing the UK defence industry and looking to ensure security of supply. "It is premature at best," said Prospect negotiator Richard Hardy.

Three years ago a move to close Bridgewater was averted with the introduction of a 10-year plan to improve business. But workers are furious that this morning’s decision was taken before the new plan has had time to provide results and that, despite assurances made at the time, jobs are once more on the line.

Hardy said: "Obviously our members are very worried about their jobs and pensions, but they are also angry that promises made a few years ago are being broken. At a strategic level we are concerned about the ability of the UK to meet its own defence requirements internally. It is particularly worrying that BAe Systems has just invested massively in US munitions production."

On behalf of 700 Prospect members in Royal Ordnance, the union plans to fight the closures and will be lobbying local MPs and the Defence Select Committee in an attempt to protect jobs. Members are also being consulted on possible industrial action.