More than a quarter of MPs has charged first-class rail travel to the taxpayer in the past year, according to latest figures.
An analysis of MPs’ expenses 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 as the Chancellor claimed that the aide had actually got into a confrontation with an inspector over whether he should have moved to standard.
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.”