Commuters could have to stump up another £664 a year for their season ticket.
The new Thameslink, Southern and Great Northern (TSGN) franchise, which is owned by Govia, is also set to take over Southern and Gatwick Express services from July next year.
And the local Labour Party believes the upshot will be increased prices for commuters having unearthed confirmation that where two existing franchises exist - like First Capital Connect and Southern on the Brighton to London line – the cheaper fare will be phased out.
As a result, the Labour Party estimates some commuters from Brighton to London will pay £4,304 a year - a rise of 18%.
An off-peak Brighton to London return could cost £28.50, rather than the current cheapest option of £16.50.
Nancy Platts, Labour’s parliamentary candidate in Brighton Kemptown and Peacehaven, said: “At the moment people have a choice and can save money by using the cheaper fares, the decision to phase out those fares will come as a nasty shock to many who can’t afford the more expensive tickets.”
A spokesman for the rail company said prices had not yet been finalised and any decision on increases would be made by the government.