MPs and peers are currently on their Easter break and were not due to return until 22 April, but both will now go back to Westminster for the hastily convened sitting.
It will be only the fifth time since World War Two that Parliament has sat on a Saturday, showing the importance the government is placing on the law.
Sir Keir said the government wanted to pass the legislation in a single day, adding the future of the company \"hangs in the balance\".
He said steelmaking was \"essential for our future\" and that he would always \"act in the national interest to protect British jobs and British workers\".
\"Jobs, investment, growth, our economic and national security are all on the line,\" the prime minister said.
The BBC understands from a senior government source the emergency legislation being voted on in Parliament on Saturday would not give ministers the power to nationalise British Steel - another bill would be needed to do that.
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