A spokesman for Virgin Trains said there is no special policy on upgrades for MPs or anyone else.
He said: "If you have got a standard ticket, you sit in standard class but there is the option to upgrade if you so choose.
"The strict letter of the law would be that if somebody had a ticket for standard, they should sit in standard and then ask the train manager to upgrade to first class and pay the appropriate charge.
"But it is not a big issue if someone sits in first class before they have paid for the upgrade.
"This is normally all done very amicably."
It follows the Chancellor George Osborne's dispute with a ticket collector after he sat in first class with a standard class ticket.
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The country's national newspapers have offered their reactions to the George Osborne ticket row, broken exclusively by Granada Reports.
Cheshire MP George Osborne has argued with ticket collectors after he refused to sit in standard class on a train to London.