As reported, about 40 jobs are at risk at its Redcar and Skinningrove plants because of weak demand in the construction industry.
About 340 jobs could be affected in Scunthorpe and 90 in Workington.
And David Hulse, national officer of the GMB union, said: "This terrible news has come straight out of the blue.
"It is a shock to the workforces in Scunthorpe, Workington and Teesside.
"GMB is bitterly disappointed as earlier this year we saw 1,000 people leave the company and everyone involved has worked hard to make all-round improvements.
"GMB will insist on no compulsory redundancies and to bring the number of job losses down."
Article form the Gazette Live