About 1,600 jobs will be lost when the plant is mothballed
The threat of industrial action at Corus on Teesside has been raised following the announcement that production will cease on Friday.
The mothballing of the Teesside Cast Products (TCP) blast furnace at Redcar means about 1,600 jobs will be lost.
Owners Tata said the decision was made as TCP's remaining iron ore is used up.
But Community Union General Secretary Michael Leahy called the move "premature" and said industrial action could not be ruled out.
The site has been at risk since last April, when international buyers pulled out of a 10-year contract to buy steel slabs.
In a statement, Indian firm Tata said: "It has not been possible to secure sufficient short-term sales of slab so as to enable TCP to operate at other than a cash loss.
"The timing of the mothballing is dependent on the consumption of the remaining iron ore stockpiles and is expected to take place later this week."
The predicted knock-on effect of the mothballing has been described as "horrendous", with Redcar and Cleveland Council estimating that up to 8,000 further jobs could be lost at surrounding companies.
It will mark the demise of 150 years of steelmaking on Teesside.
Mr Leahy said: "We still believe a viable deal can be done with a potential buyer. The slab market is improving week by week and we are firmly of the view that Tata Corus have acted too quickly.
"By putting profit before the people of Teesside, Tata Corus have undermined the foundation for future growth. We will continue to fight for a future for Teesside.
"Lots of questions remain unanswered, not least the length of time that the mothball will take place.
"In view of the manner in which this has been done, with Tata Corus riding roughshod over any consultation, it means we cannot rule out seeking our members' support for industrial action."
Business Secretary Peter Mandelson has pledged £60m to help the area recover when the plant closes, but many argue the cash should instead be spent on trying to save the steelworks.