Peers gave final approval to the Channel Tunnel Rail Link (Supplementary Provisions) Bill, which had already cleared the Commons. It now awaits Royal Assent to become law.
Rail minister Tom Harris said, during the Bill's earlier Commons stages, that the "High Speed One" link would slash journey times for Kent commuters from December 2009 with a new fleet of trains.
The line would now be auctioned off, as the London and Continental Railways consortium in charge of building the record-breaking line was broken up.
Mr Harris told MPs then that there would be an "open, competitive process for any sale, to secure best value for the taxpayer". The Bill was the "first step towards this," he added.