The Government has invited bids for the UK's 40% stake in the Eurostar to boost public finances.
The plans, announced last year, hope to raise £20 billion from corporate and financial asset sales by 2020.
Chancellor George Osborne said potential bidders should respond by the end of the month.
He added: "I am determined that we go on making the decisions to reform the British economy and tackle our debts so we will proceed with the potential sale of the UK's shareholding in Eurostar today.
"Ensuring that we can deliver the best quality infrastructure for Britain and the best value for money for the taxpayer are key parts of our long term economic plan."
The decision has been attacked by rail unions. While Labour warned it could follow the Royal Mail privatisation as a "rushed and under valued" sell off.
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