Steel production at the Corus plant on Teeside will cease within a week, staff have been told.
Teesside Cast Products (TCP) blast furnace at Redcar, has been at risk since potential buyers withdrew from a 10-year contract. Mothballing will begin on Friday, with the loss of around 1,600 jobs, according to the BBC.
This had already been deferred by a month, following union negotiations.
Unions said the site was still viable, but Nick Brown, minister for the North East, described the chances of finding a new buyer as a long-shot.
Redcar and Cleveland Council said the knock-on effective for the area would be horrendous, and has estimated that up to 8,000 more jobs could be lost at surrounding companies.
The closure marks the end of 150 years of steelmaking on Teesside.