Research carried out by Campaign for Better Transport warned that tickets from Brighton to the capital will significantly rise once the daily paper ticket system is removed.
Norman Baker, from Campaign for Better Transport, said: "Withdrawing the day travelcard will not only remove access to integrated, seamless ticketing for millions of people, but also make it much more expensive to visit the capital.
"This move is going in entirely the wrong direction. Public transport works best when you can use one ticket for your whole journey, so we need more, not less, integrated ticketing."
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The day travelcards will be withdrawn next year, after Transport for London (TfL) said it could no longer afford to fund the travel agreement.
The ticket allow people to travel on the Tube, bus, rail, tram, DLR and London Overground for a singular fee.
Currently, an adult anytime day single from Brighton to London Victoria costs £31.90, a return would set you back £62, while the return travelcard is £70.
TfL said the decision "remains reversible" and that it wanted to explore options to continue providing the day ticket.
TfL estimates it can bring in as much as £40 million if pay as you go is used instead.
A total of 12 million day travelcards were sold last year. It is nearly double the amount sold during the Covid-19 pandemic but down from 27 million in 2018.