Iron and steelmaking at SSI's plant in the north-east of England is to be mothballed, with the loss of 1,700 jobs.
Production at the Redcar plant was paused on 18 September with the company citing poor steel trading conditions and a drop in world 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 described it as "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.
'Absolutely heartbreaking'
Iron ore was first discovered on Teesside in the 1850s. The steel works, which are being mothballed, were founded in Redcar in 1917
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.
Labour MP Ms Turley accused SSI of playing "fast and loose" with the plant's future, and claimed the government could have done more to prevent the closure.
She said: "It's absolutely heartbreaking, it's devastating.
"It's a huge number of jobs, it's a huge number of livelihoods, but it is more than that to our community.
"Redcar was built on steelmaking, Teesside was built on steelmaking. It's about generations of people that have worked in that blast furnace and fought so hard, not just in the last few days but in the last few weeks, months, years, to keep steelmaking alive. It's just been allowed to fizzle out."
She called for the government to take control of the site and its assets to ensure a future for steelmaking on Teesside and in the UK.
Business Minister Anna Soubry said: "Despite everyone's recent efforts to help SSI this is very sad news and a big blow for the workforce and their families."
'Challenging' conditions
She said a taskforce had been set up to support workers.
"I hope that SSI's announcement that they are mothballing the furnace gives some hope that steelmaking could be restarted on Teesside in the future", she said.
"The steel industry across the UK is facing very challenging economic conditions. The price of steel has almost halved over the past year, with over-production in the world market.
"While government cannot alter these conditions, I have called a steel summit to see what more can be done to help our steel industry."
Linda Robinson's family has been involved in steel manufacture for generations.
She said: "Every family on Teesside knows somebody in the steel industry. And we've got near 170 years of steel making.
"We're the original northern powerhouse, forget this new title. We demand that they save the steel works, forget mothballing. We deserve this."
'Vital' skills
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.
General secretary Roy Rickhuss 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."