'Narrow' South West Trains' seats criticised
South West Trains has been criticised for introducing seats with a width of 43cm (18in) on some of its mainline trains between Portsmouth and London.
The firm said it meant it could provide dozens more seats to help ease overcrowding problems.
But Portsmouth North MP Penny Mordaunt said many passengers had complained the seats had left them in "agony".
She raised the issue in the House of Commons and called for minimum standards in comfort on trains.
The Conservative MP said: "What's happening is that trains designed for suburban routes, and designed for people to be on them for 20 minutes or 30 minutes maximum, are being used on mainline services.
"So people commuting from Portsmouth might be on them for nearly two hours and it is really agony for them.
"I'm getting a lot of complaints from people suffering back injuries, leg problems.
"What we want to do is ensure that in future rail franchises have minimum standards of comfort for passengers."
The MP admitted customers were often unhappy by the lack of seats but said the same people were just as concerned about comfort.
The new seat sizes have been introduced on the firm's Class 450 trains which means the carriages can now fit five seats across instead of four.
A spokeswoman for South West Trains said: "Class 450 trains are modern, reliable, air-conditioned rolling stock and every aspect of these trains, including the seats, has to comply with strict legislation and Railway Group standards.
"In addition, an independent assessment of the seating design and layout demonstrated that the seats on the Class 450 are ergonomically sound.
"Our customer feedback shows that getting a seat is high on the list of priorities for our passengers - these trains, when run as 12 cars, provide 142 additional seats per journey compared to a 10-car Class 444 train."
Passenger Focus said: "The longer-term solution is to increase capacity, more rolling stock is needed so that we can balance comfort and capacity."
Transport Minister Theresa Villiers has agreed to meet Ms Mordaunt to discuss the issue.