More than a quarter of MPs have charged first-class rail travel to the taxpayer in the past year, it emerged today.
An analysis of MPs' expenses by The Sunday Telegraph found that 185 had claimed for first-class train tickets.
First-class travel has been curbed by the Independent Parliamentary Standards Authority (Ipsa), brought in after the 2009 expenses scandal to sweep away the previous discredited regime.
However, it is still permissible in circumstances where it would be cheaper than buying a standard open ticket.
The extent to which MPs are taking advantage of that rule was revealed after Chancellor George Osborne faced ridicule after entering a first-class carriage yesterday with only a standard ticket.
His office insisted he had always intended to pay for the upgrade and that an aide had sought out the train manager to do so. However, a television reporter travelling on the same train claimed that the aide had actually got into a confrontation with an inspector over whether he should have moved to standard.
According to The Sunday Telegraph, some of the first-class ticket claims have cost as much as £300, five times as much as the cheapest standard fare for the same route.
MPs who have travelled first-class on expenses include the Transport Secretary Patrick McLoughlin, Cabinet Office minister Francis Maude, Transport Minister Norman Baker, shadow chancellor Ed Balls and former chancellor Alistair Darling, the paper said.
In total there were 113 Labour MPs, 48 Conservatives, 19 Liberal Democrats, two Plaid Cymru and three Scottish National Party who have claimed for first class rail travel in the past year.
Ipsa's guidelines on travel expenses suggest MPs should consider "value for money" and whether cheaper, inflexible tickets will end up costing more if travel arrangements change at short notice.
"You can claim for first class travel if it is less than the cost of a standard open fare," it states.
First-class tickets purchased far enough in advance can end up cheaper than open standard tickets bought shortly before the journey.
But Taxpayers' Alliance chief executive Matthew Sinclair said: "If MPs can get themselves organised to order a first-class ticket in advance, they should be able to order a standard-class ticket in plenty of time as well and it will almost always be cheaper.
"If standard-class travel isn't good enough for MPs, it isn't good enough for ordinary commuters who pay for their own tickets."
Education Secretary Michael Gove admitted having upgraded to first class but insisted that a minister doing so and paying "out of his own pocket" was not a story.
"I have done what lots of people will do in such circumstances which is take advantage of what used to be called 'weekend first' I think, which means that you can pay £5 or £10 or £15 in certain circumstances at the weekend, when you have got a standard class ticket, to travel in first class," he told Sky News.
"Sometimes when you are travelling with your family and sometimes when you have got work to do that additional extra payment is worthwhile.
"I don't think I have ever paid as much as George paid - out of his own pocket - to upgrade."