Redcar Steelworks has dominated the Teesside skyline for decades.
1,700 jobs at Corus are to go, as the decision to mothball the plant is made at 10am on Friday, 4 December 2009.
Union leaders said the news was "devastating" and called on the Government to take immediate action to help save the jobs.
The future of the plant was put in doubt earlier this year when its main customer ended a purchasing agreement.
The Teesside Cast Products factory had a 10-year contract to supply slab steel to a consortium of four buyers.
A Corus spokesman said: "This is the partial mothballing of the TCP plant. The company is shutting its blast furnace, its steel melt shop, which makes slab and one of the two coke ovens.
"They will be keeping open the Redcar Wharf facility which handles imports and shipping, one of the two coke ovens and some of the power generation capacity.
"The total loss will be 1700 jobs. Because its only a 'partial mothballing' the number of job losses will be 600 fewer than envisaged last May when they said there were 2300 jobs at the TCP plant.
"Also retained in the area will be the Teesside beam mill as well as 'other large facilities, which means Corus will remain a substantial presence in Teesside with at least 2000 people employed there'."
'Gut wrenching disappointment'
Vera Baird QC MP said:
"I know this is a gut wrenching disappointment in the sheer dogged way the workers picked it up after the consortium walked off in April.
They won good quality export orders but it seems Corus nationally could not turn it into a sale.
"I'm very upset by this news and my heart goes out the people who have received this news two weeks before Christmas."
Help for workers
George Dunning, Leader of Redcar and Cleveland said:
"The Government need to realise that this is ripping the heart out of the manufacturing industry on Teesside, and could spread across the region.
"The council have been talking to the steelworks and have things in place to help the workers.
"We have a room in Steel House to help people to apply for benefits and our officers are all ready to go, but it's small change in a bigger picture."
Find alternative business
Business Secretary Lord Mandelson said:
"We have been working on this all year to try to find an agreement to reinstate the contract.
"We are not going to walk away from these people that have lost their jobs, and will do everything we can to help the individuals concerned, but what we need to do is find alternative business.
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