The train firm which runs services between the Westcountry and London has been accused of "gaming" the system without facing a penalty.
Shadow Transport Secretary Maria Eagle pointed to FirstGroup's decision to end its contract for the Great Western line three years early, and so avoiding making an estimated £800 million in payments to the Government.
She highlighted it as a further flaw in the franchise process, which is already facing calls for reform after the West Coast rail line fiasco.
The costly debacle has meant the new Great Western deal, due to start next year, is being put on hold pending the outcome of an independent inquiry into contracting arrangements.
First Great Western has insisted it is meeting the commitment made with government.
Ms Eagle is pressing for the government review to consider changing the franchise system, arguing the current one had failed to deliver the promised benefits.
And she did not rule this out including putting the running of the railways back into public hands.
One of her criticisms was the so-called "gaming" of the system by operators, without being penalised.
Ms Eagle said: "Just to remember, the reason why Great Western is coming up now is that First could have run it for another three years had it chosen to do so.
"But they handed it back, in order to avoid making £800million of payments to Government and there doesn't seem to have been any kind of penalty, or debt, for them for doing that.
"That's another aspect of this franchising process and the way it's run which is unsatisfactory from a taxpayers' point of view," she continued.
"There's been no penalty for gaming the system.
"I'm not accusing anybody of engaging in unlawful behaviour, or anything like that, but they have been gaming the system, and there's been no penalty for it."
A FirstGroup spokesman said: "We are fulfilling our commitment as agreed with Government. We announced in May 2011 that we would not be taking up our option to extend the First Great Western contract for a further three years beyond the initial franchise term to 2013.
"We will continue to operate First Great Western until the current franchise ends and deliver for our customers."