About 17p of every pound earned in low-income households that own a car is spent on motoring.
A report commissioned by the RAC Foundation said the economic group, half of whom now own a car, will be the most affected by any Government attempts to extract more money from the driving public.
The 'Low Income Motoring in Great Britain' report revealed that the lowest-earning fifth in the car-owning households list pay more as a proportion of their income on purchasing and maintaining a vehicle than any other section of society.
Already, the least well-off drive the oldest and cheapest cars, the report added, making it very difficult for a 'trade down' during a time of financial problems.
RAC Foundation director Professor Stephen Glaister said: "Running a car is absolutely vital for the majority of people in this country. And whilst the wealthier members of society spend more on motoring in absolute terms, it is the poor who pay most as a proportion of their incomes."
"Whilst ministers are urgently looking for ways to fill their financial black hole, they must resist the temptation to raise motoring charges through the roof.
"These already unfairly burden the least well-off, and increases will affect them disproportionately. Given how much those on the lowest incomes spend on cars, and how reliant they are on them, it would be grossly unjust to penalise them further."
Copyright © Press Association 2009