Amicus said it was also now their priority to make sure none of the staff affected accept redundancy until the next generation Astra model is secured for Ellesmere Port.
Derek Simpson, general secretary of Amicus, said: "This is another devastating blow to the car industry and UK manufacturing in general.
"Our priority now is to make sure that there are no compulsory redundancies made at the plant and we are committed to ensuring that the Astra replacement comes to Ellesmere Port to ensure a long term future for the people that are staying."
Amicus is blaming weak UK employment laws for the job losses that saw the UK plant, the most productive of all GM’s European plants, cut ahead of any of the other European plants.
Tony Woodley, T&G secretary (a former worker at Ellesmere Port), said: “There is no doubt this cut to a productive and successful plant, GM's most improved plant in Europe, is being made because GM can sack our people on the cheap."
"This plant, our industry and our country cannot afford to keep shedding skills like this," he said.
The T&G welcomed the support of Chancellor Gordon Brown for the future of the plant. Brown said the Government would do all it could to support the plant winning that new model Astra.