Prime Minister David Cameron and Labour leader Ed Miliband traded accusations today over who was responsible for rail fare hikes of up to 11 per cent.
Both men faced calls to apologise for misleading the Commons during the furious exchange at Prime Minister's Questions.
Mr Miliband blamed the Government for the scale of fare rises on the most popular services this month, saying the coalition had brought back flexibility on prices after the last Labour administration scrapped it.
But Mr Cameron shot back that Labour had only got rid of flexibility - allowing companies to increase fares by more on the busiest routes - for one year in 2010.
"They changed it for one year only, an election year, with no intention of making it permanent and if he doesn't know that he should," Mr Cameron said.
But the Labour leader insisted the Prime Minister was "wrong".
"The last Labour government saw that train companies were taking advantage of consumers, ripping them off, by increasing fares more on the busiest routes and stopped it. We took away that power from them," he said.
"You came to office and you brought the power back, you made the wrong decision."
The Government later produced a Department for Transport contract stating that the ending of flexible fares would be "reversed" on January 1 2011.
But Labour peer Lord Adonis, who was transport secretary at the time, said it had been his "firm intention" to continue with the restrictions after 2010.
"The ending of the flexibility was strongly opposed by the train companies, but they complied and it was highly popular with passengers.
"It was my firm intention to continue the policy for subsequent years, and I was mystified when Philip Hammond, my successor, reinstated the fares flexibility."
His comments appeared to be supported by a transcript of a 2009 appearance before the Commons Transport Committee, when he said: "The Government's intention is that in future the cap should apply to individual regulated fares, not just to the average of each fares basket."
But Downing Street said it was "very clear in the agreement with the train operating companies that it was for a single year".
The Prime Minister's spokesman acknowledged that it was open for the coalition to renew the agreement after it lapsed on January 1 2011.
But he added: "There is a cost associated with that and, as the Prime Minister said, the money for trains has to come from one of two places. It comes from fares or from the taxpayer.
"If you had extended that for another year, there would have been a cost. You could have got that from taxes or you could have let the deficit go up. These were your choices."
The spokesman denied that the decision not to repeat a one-year arrangement amounted to a change in the policy inherited from Labour.
"The accusation in the House of Commons was that we changed policy," he said. "We accepted the policy and the policy was that it was in place for one year."
Conservative Party co-chair Baroness Warsi called on Mr Miliband to apologise to Parliament.
"Ed Miliband must have known that ministers he sat in government with had signed an agreement with the rail companies that explicitly reversed the changes it made on 1 January 2011," she said.
"Now that the evidence is there in black and white for all to see, Ed Miliband should correct the record and apologise to Parliament.
"Either he was misleading the House or he simply didn't do his homework."
But Labour MP Simon Danczuk accused the Prime Minister of lying and said it was for him to apologise.
"David Cameron is telling a lie about Labour because he is embarrassed by his failure to stand up to the train companies. He is blushing red about the way Ed Miliband is standing up for passengers and the squeezed middle.
"David Cameron knows the truth and he should come back to the House, apologise and immediately correct the record."