Iron and steelmaking at SSI's plant on Redcar is to be mothballed with the loss of 1,700 jobs.
Production was paused on 18 September amid poor steel trading conditions across the world and a drop in steel prices.
SSI UK business director Cornelius Louwrens said: "This is an extremely sad day for all of us at SSI UK."
Redcar MP Anna Turley said the announcement was "devastating news".
SSI said it had carried out an assessment of the business situation it faced and had concluded there was "no other option" but to mothball its iron and steel making facilities on Teesside.
The Redcar coke ovens and the power station will continue to operate.
It said it would continue to work with stakeholders, including the government, to look at restarting operations at some point in the future.
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The plant was mothballed by previous owner Tata Steel in 2010. SSI spent millions of pounds making the furnace ready again and it was relit in 2012.
'Chance of a future'
The GMB union said it would be calling on the government to take action.
Its national officer, Dave Hulse, said: "The government has got to take some responsibility for this.
"They can still intervene to save Redcar and the thousands of jobs that rely on the plant."
The Community union said it would be calling an urgent meeting with SSI, and would be supporting its members.
General secretary Roy Rickhus said: "Steelmaking on Teesside must have a future and our fight to save our steel will continue.
"These vital industrial skills have been passed down through the generations and must not be lost.
"That blast furnace must not become a monument to 170 years of history and it must be mothballed properly to preserve its integrity and give it the chance of a future."