Iron and steelmaking at SSI's Redcar plant in the north-east of England is to be mothballed, with the loss of 1,700 jobs.
The company "paused" production on 18 September citing poor trading conditions and a drop in world steel prices.
SSI UK business director Cornelius Louwrens said the plant could be mothballed for up to five years.
Redcar MP Anna Turley described it as "devastating news".
SSI said it had carried out an assessment of its business situation and concluded there was "no other option" but to mothball its iron and steel making operations on Teesside.
Mr Louwrens said: "This is an extremely sad day for all of us at SSI UK, and in particular our employees and their families."
He said he did not know how long the site would be mothballed and the price of steel would need to recover before there was any prospect of the site reopening.
See reaction to the news on the BBC North East Local Live page.
The company revealed about 450 people would be kept on but 1,700 workers would go.
The Redcar coke ovens and the power station will continue to operate.
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 on the government to take control of the site and its assets to ensure a future for steelmaking on Teesside and in the UK.
But Conservative MP James Wharton, who is the Minister for Local Growth and the Northern Powerhouse, rejected a call for the plant to be renationalised.
He said: "An industry which has made a loss over each of the last three years would pass that responsibility to the tax payer.
"It would present huge issues for the rest of the steel market in this country in terms of how they would then be able to compete with, what will be, a nationalised company.
"It's just not something that is going to happen."
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."
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."
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."
Iron ore was first discovered on Teesside in the 1850s and the steel works, which are being mothballed, was founded in Redcar in 1917
The plant was mothballed by previous owner Tata Steel in 2010. SSI then spent millions of pounds making the furnace ready again before it was relit in 2012.