The government has put major projects not under way on hold
Cuts to transport budgets could result in rises in rail fares, more potholes and fewer buses, campaigners say.
The Campaign for Better Transport (CBT) says the transport budget is not protected and could be reduced by 25%.
The government says it is to suspend schemes not yet under way until the outcome of its autumn spending review for the four years from 2011.
The charity says the approach of "salami slicing" across the board will bring overcrowding and fare increases.
In a draft submission to the government's spending review, the CBT says a better approach would be to find the savings by stopping major road building projects, such as the £1.3bn extension of the A14 in Cambridgeshire.
CBT executive director Stephen Joseph said: "Our research shows that on transport spending the government has a choice.
"It can go for easy cuts, taking money out of all budgets, upping fares and giving priority to big roads. Or it can go for smarter cuts which protect the spending that helps the economy, meets carbon and environmental targets and strengthens communities."
He said existing train services should be protected and enhanced but there was a danger that if significant cuts were made in the wrong places, "there will be increased overcrowding, fares rises and reduced or slower services".
Mr Joseph added that road building projects were poor value for money and maintaining, and making better use of, existing roads should be the priority.
He said: "There is a real danger that buses will become extinct outside the centres of cities and towns, as funding for them will be under threat."
The government has already said spending at the Department of Transport this year is be reduced by £683m, with cuts in grants to local authorities amounting to £309m.