Virgin Trains' decision to stop selling the Daily Mail on its West Coast services has been described as "absurd" and "pompous" by Boris Johnson.
The foreign secretary said the actions of the firm, which in a memo declared the paper "not at all compatible" with its brand, resembled censorship.
A Daily Mail spokesman has called the decision "disgraceful".
Labour said it would not restrict sales of the paper, if it succeeded in renationalising the railways.
A spokesman for Jeremy Corbyn added the Labour leader was an "enthusiastic supporter of a free press and the pluralism of the press.
"Obviously private companies will decide what they want to stock on their trains."
A Downing Street source said the move was ultimately a decision for Virgin Trains but Prime Minister Theresa May had "always been clear on the importance of a free press to our democracy".
Meanwhile Mr Johnson, a former journalist, tweeted: "Absurd ban on Daily Mail by Virgin! Pompous, censorious and wrong #virginontheridiculous".
The Society of Editors has also said it is concerned about the decision taken by Virgin Trains as it "smacks of censorship".
Listened to feedback
Virgin West Coast runs trains between London, the Midlands, the north-west of England and Scotland.
Virgin said on Tuesday it regularly reviewed products sold on its trains, and "after listening to feedback from our people" decided to stop stocking copies of the Daily Mail in November.
In an internal memo to staff explaining its decision not to sell or give away the paper, Virgin said there had been "considerable concern" raised by colleagues about its editorial position on issues such as immigration, LGBT rights and unemployment.
"We've decided that this paper is not compatible with the VT brand and our beliefs," it said.
"This won't suit all of our customers or all of our people – it's certain to draw some criticism. But we've listened to many colleagues over the last few months, and we feel that this is the right move to take."
The Daily Mail claimed that only the Daily Mirror, Financial Times and the Times would now be on sale on Virgin Trains.
The Mail said all three papers had backed the views of the train company's owner, Sir Richard Branson, in backing the Remain side in the EU referendum, a stance it opposed.
A Mail spokesman added: "It is disgraceful that, at a time of massive customer dissatisfaction over ever-increasing rail fares, and after the taxpayer was forced to bail out Virgin's East Coast mainline franchise, a decision strongly criticised by the Mail, that Virgin Trains should now announce that for political reasons it is censoring the choice of newspapers it offers to passengers."
Virgin Trains said the Daily Mail had never been stocked on its East Coast trains run under the management of Virgin/Stagecoach.