Steel production at the Corus plant on Teesside will cease within a week, workers have been told.
The mothballing of the Teesside Cast Products (TCP) blast furnace at Redcar, where about 1,600 jobs are due to be lost, will begin on Friday.
TCP has been at risk since international buyers pulled out of a 10-year contract.
Unions said the site was "still viable" and they would not give up hope that a buyer for the plant could be found.
The mothballing had been due to start at the end of January, but following a meeting with union leaders the firm agreed to extend operations to the end of February.
In the meantime, talks have been ongoing to secure a "positive future" for TCP.
However, the firm has now confirmed that the manufacture of steel slabs will come to an end within days.
The knock-on effect for the area 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.
Business Secretary Peter Mandelson has pledged £60m to help the area recover, but many argue the cash should instead be spent on saving the steelworks.
Local MPs raised the issue in Westminster Hall: From BBC Democracy Live
Geoff Waterfield, multi-union chairman at TCP, said: "I want somebody with a bit of common sense to see those works for what they actually are - a very good investment.
"Not only is it going to make profit for whoever owns that company, but it's going to make sure a community and region survive.
"I'll continue to fight, as everyone else in those works will."
Last month Nick Brown, Minister for the North East, said the chances of finding a new buyer for the site were a "long shot".
More than 1,700 workers had been at risk until January, when a deal was reached to retain TCP's South Bank coke ovens, safeguarding 120 jobs.