Labour is urging the Government to delay signing a new West Coast Mainline contract so MPs have a chance to fully scrutinise the deal.
Maria Eagle, the shadow transport secretary, has said the decision to take the franchise from Virgin Trains and award it to FirstGroup should be put on hold until Parliament returns next week.
The politician said there were concerns over how the move would affect fares and levels of service.
Her plea comes after Louise Ellman, the chairman of the House of Commons Transport Committee, wrote to Transport Secretary Justine Greening asking her to hold off signing the final contract, saying that "important issues" had been raised.
More than 100,000 members of the public have also signed an online petition against the decision, in a campaign supported by double Olympic champion Mo Farah, Apprentice star Lord Sugar and celebrity chef Jamie Oliver.
Virgin boss Sir Richard Branson has offered to run the line for free to allow Parliament time to debate the issue.
The entrepreneur, who has claimed that FirstGroup's bid will lead to "almost certain bankruptcy", on Sunday made a last-ditch appeal to the Government to delay signing the 13-year contract on Tuesday.
He said Virgin Trains and Stagecoach would operate the joint venture on a not-for-profit basis or donate profits to charity if the franchise needed to be extended beyond December for a few months to allow Parliament to investigate the decision.
Sir Richard is also pressing for an independent audit of the Department for Transport's decision over the £10 billion deal.
Sir Richard said: "It is far better for MPs to have the chance to debate the issues, and question ministers on the detail before the decision is finalised.
"To assist in this process, there should be an independent audit of the DfT decision to ensure it has been based on correct criteria and reliable forecasting of customer numbers, revenue and payments to Government.
"We must ensure that this crucial decision is taken with all the facts correctly assessed and understood."
Virgin has operated the West Coast line since 1997 and has more than doubled annual passenger numbers over 15 years.
FirstGroup claims it will deliver better value for taxpayers.
It plans major improvements to the InterCity West Coast franchise to enhance the customer experience, including improved wifi and catering, as well as additional services and more seats and reducing standard anytime fares by 15% on average.