A rail passenger watchdog has called on a train company to explain why it is raising some off-peak fares by as much as 20% later this month.
SWT said the prices would bring it into line with other operators
The increases, to be brought in on 20 May by South West Trains (SWT), were unacceptable, Passenger Focus said.
It added that it wanted SWT passengers to give their views on the rises.
An SWT spokeswoman said the new prices would bring the company into line with other operators and allow it to "better match demand and pricing".
Examples of the increases, which were first announced in March, include a cheap day return from Alton, in Hampshire, to London rising from £15.80 to £19 - a 20.25% increase.
A Bournemouth to London cheap day return will go up from £36.40 to £43.70 - a 20.05% rise.
Anthony Smith, chief executive of Passenger Focus, said: "These increases are unacceptable and come only a matter of months after their last price rises.
"The affordable, turn-up-and-go railway has been further eroded and SWT is exploiting a monopoly market.
"Families making day trips to London will be hit hard by these changes. We want SWT passengers to tell us what they think of the fare increases and how they will be affected."
An SWT spokeswoman said: "This pricing structure will bring us into line with a number of other train companies and allow us to better match demand and pricing.
"People who decide to travel outside of the peak can still make really good savings by travelling a little later or by buying one of a range of off-peak value tickets, megatrain.com deals and by using discount Railcards."