The company said passengers are reserving seats on different services
A train operator is to begin charging passengers for reserving a seat.
National Express is to charge £2.50 for reserving a single, or £5 for a return ticket, from this weekend on its East Coast and East Anglia franchises.
The Transport Salaried Staffs Association said the move was "outrageous" and warned it could spark abuse against booking office staff.
The firm said some passengers reserved seats on different trains and it wanted to ensure they would not be left empty.
A Transport Salaried Staffs Association spokesman said it was the highest seat booking charge ever imposed, adding that the move came at a time when the East Coast franchise holder was reportedly trying to renegotiate its contract with the government.
The union said that booking clerks had told it they were worried about the reaction from passengers.
"All staff will face regular abuse, especially when passengers are spending £223 for an open ticket and then have to pay £5 on top," an internal memo from members in York said.
Gerry Doherty, the union's general secretary, said: "This is an outrageous imposition on millions of passengers.
"National Express is now saying to passengers if you want to be sure of sitting down on their trains then you will have to pay an extra £5 for a return journey or £2.50 for a single.
"It will hit the elderly and families the hardest."
National Express confirmed the charges will be introduced from this Sunday on some seat reservations on its East Coast route between London and Edinburgh and East Anglian routes through Cambridgeshire, Essex, Hertfordshire, Norfolk and Suffolk.
A spokesman said: "If a customer does not want to reserve a seat, there is no charge.
"This does not apply to any advance purchase tickets, first class tickets, season tickets, customers with a disabled railcard or assisted passengers.
"We find that people are often reserving multiple seats as they're not sure which train they are going to catch.
"By asking people to pay for a seat reservation, seats will no longer be left empty with a reserved sign, therefore being made available for other customers to use."