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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