Teesside Cast Products will continue to make steel until the end of February or until raw material has run out, it has been revealed.
The plant, based in Redcar, was due to be mothballed in January with the loss of about 1,700 jobs.
Community Union said the decision would provide time for an alternative for the future of steelmaking on Teesside.
Earlier in the week the firm agreed a last-minute deal to keep the South Bank Coke Ovens open, safeguarding 120 jobs.
Community Union's general secretary Michael J Leahy said: "These additional weeks will provide valuable time to find an alternative future for steelmaking on Teesside.
"Tata Corus need to be open-minded to all possibilities as the future of an entire community depends on their actions in the coming weeks."
If the plant was to close, it is estimated that indirectly, around 4,000 jobs would be affected - around 2,000 of those would be from the Teesside area.
A further 1,000 contractor jobs could also be affected.
On Friday the North East Region Parliamentary Select Committee met to discuss the plant's future.
MPs asked what was being done to save jobs at the Teesside Cast Products plant in Redcar.
It was not attended by Corus chief executive Kirby Adams.
The select committee chairman Dari Taylor MP invited him to make a future date.
She said: "There was not one banker that refused to meet the relevant select committee during this recession; we really hope that Kirby Adams is not refusing to present evidence to the committee."
A further evidence session will take place at the House of Commons on Tuesday, and Ms Taylor said the inquiry would also examine whether the government was doing everything it could.