Transport Secretary Justine Greening has told the BBC there will not be a delay in signing a new West Coast Main Line contract with FirstGroup.
Mrs Greening said the government would "push on" with the 13-year contract, which was a "good deal for taxpayers".
There has been pressure from Virgin Rail, which currently runs the line, and from Labour, to delay the decision until Parliament returns.
The chairwoman of the Transport Select Committee has also called for a delay.
The contract signing could happen on Wednesday.
FirstGroup has said it would bring in key improvements for passengers, but critics fear it would not be able to afford the £5.5bn it was paying for the franchise.
Virgin's Sir Richard Branson had offered to run the service "for free" while a review was carried out.
But Mrs Greening told BBC Breakfast that all bidders had "bought into" the "fair and well-established process". She claimed that if Virgin had won the bid it would have "been perfectly happy with the process".
Service improvementsAberdeen-based FirstGroup already operates a number of rail routes, including Great Western and ScotRail.
The company, under the name First West Coast Limited, will take over the franchise from 9 December and is due to to operate the service until 2026.
The West Coast Main Line route serves 31 million passengers travelling between London, the West Midlands, the North West, North Wales and the central belt of Scotland.
Virgin has run the franchise since 1997, during which time passenger numbers doubled.
FirstGroup has promised to introduce better wi-fi and food, more frequent trains and more seats, and to cut standard fares by 15%.
The firm said it would introduce 11 new 125mph six-car electric trains on the Birmingham to Glasgow route and provide more direct services between destinations.
PetitionMore than 100,000 people have signed an online petition calling for the decision to be reconsidered.
Labour's Maria Eagle, the shadow transport secretary, has urged the government to put the contract on hold until MPs return from summer recess and have a chance to consider the deal.
And Labour's Louise Ellman, who chairs the House of Commons transport committee, has written to Ms Greening asking her to delay signing the contract.
On Sunday, Sir Richard said that if reviewing the decision meant the December deadline had to be postponed, his company and Stagecoach would be willing to continue operating the railway lines while donating any profits to charity.
Sir Richard told the BBC that he wanted a proper examination of the facts.
"We've asked 40 questions - not one of them have been answered. What we want is transparency," he said.
