Passengers travelling on the busiest trains between Birmingham and London might only be allowed to board if they have booked seats in advance, Virgin Trains has said.
The operator has set out plans to drop walk-on fares on the West Coast Main Line which serves New Street station to deal with overcrowding.
Hundreds of people are regularly turned away from Euston station on Friday evenings as thousands try to pile on to the three-times-an-hour services.
Virgin said it can no longer carry all the passengers that want to travel at peak times on the line, which carried 28 million passengers a year.
A spokesman for the firm said it had not ruled out sharp price hikes on the busiest trains to keep passenger numbers down.
Although the most packed trains leave Euston to head north, the firm says commuters travelling south from Birmingham could be hit by new rules on morning services.
The news came as a Parliamentary committee this week warned that the Government will miss all targets for making more room available on trains by 2014, despite the fact that fares are set to rise by three per cent above inflation.
Virgin Trains’ head of communications Richard Evans said: “There is simply no room on some of out busiest trains and we have to look at ways of solving that problem.
“There is a limited number of ways that we can address it.
“One of them is raising the prices but we would rather not do that to our passengers.
“The problem exists mainly on northbound Virgin services from Euston but also on certain trains from Birmingham where a lot of passengers are left standing.
“We want people to know that if they are going to get on a train that there will be room for them or we could start to lose business to our rivals.”
Virgin Trains asked the Government for a three-year extension to its franchise to operate the line which would have enabled it to invest in extra carriages but was rebuffed.
Its lucrative deal to operate the route expires in 2012 and other operators are keen to win the contract.
“Because the carriages are specially designed to run with the Pendalino trains, we can’t simply conjure carriages from thin air,” added Mr Evans.
Critics have said that introducing reservation only services would mark a watershed for rail operators and warned that it could price the poorest travellers off trains.
Anthony Smith, chief executive of independent rail watchdog Passenger Focus, said: “By moving to reservation only, we are starting to chip away at Britain’s great railway.
“Not only might it change the nature of travel, but it could alter the type of person who is allowed to travel.”