Virgin Trains has managed the West Coast Mainline, which runs trains between London and Glasgow, for 15 years, but last week the Government controversially awarded the new contract to FirstGroup (Other OTC: FGROF.PK - news) .
Labour says MPs should get the chance to scrutinise the deal amid concerns FirstGroup's bid is unachievable.
In a letter to Transport Secretary Justine Greening, Labour's shadow Maria Eagle said: "The decision appears almost exclusively a 'bottom line' one, driven by a particularly high pledge of payments to government.
"There are concerns about the impact of this decision on fares and levels and quality of services. I would therefore urge you to delay any signing of this contract."
An e-petition calling for a debate in Parliament over the franchise has over 100,000 signatures, and has been backed by a number of celebrities.
The Apprentice's Lord Sugar, Jamie Oliver and the comedian Eddie Izzard are among those calling for ministers to reconsider its decision to award the contract to FirstGroup.
Sir Richard Branson, the founder of Virgin Group, described the petition as "a great boost" for Virgin Train employees.
"It is clear customers want the decision reconsidered and Parliament should have the opportunity to debate this before any contract is signed," he said.
A FirstGroup spokesman said: "We appreciate that Sir Richard Branson is a high-profile celebrity with millions following him personally on social media. However, our business is transport - first and foremost.
"We are introducing 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.
"We have a long and proven track record in running rail services and will be delivering better value for taxpayers. We look forward to welcoming all customers to their new and improved service from December."