West Coast mainline operator Virgin Trains has started court proceedings over the Government's decision to award a new 13-year West Coast franchise to transport company FirstGroup from December.
In a statement, Virgin said: "We have tried for three weeks to get clarity over the DfT's decision and to have a number of key questions answered. On each occasion we have been refused information.
"We are left with no choice but to commence court proceedings as we believe the procurement process has ignored the substantial risks to taxpayers and customers of delivering FirstGroup's bid over the course of the franchise."
The Transport Secretary Justine Greening had been due to sign off the contract this week.
She said she was "disappointed but not surprised" by Virgin Train' action.
Virgin is now hoping that its legal challenge will delay the signing.
Virgin boss Sir Richard Branson has expressed concerns that FirstGroup would not be able to live up to its financial and service-providing targets for West Coast.
Today, in a statement, he said:
Responding to news of the legal challenge, FirstGroup said in a statement:
Earlier today, Justine Greening told BBC Breakfast that the West Coast Mainline contract will go ahead to take the franchise from Virgin Trains and award it to FirstGroup.
Labour has urged the Government to put the decision on hold until Parliament returns next week so MPs have the chance to fully scrutinise the deal.