The least well-off families are slipping further into "transport poverty", according to an RAC Foundation survey.
The poorest car-owning households spent at least 31% of their disposable incomes on buying and running a vehicle in 2012, the foundation said, up from 27% the year before.
The figures - based on data obtained from the Office for National Statistics - showed that in 2012 the poorest families had a maximum weekly expenditure of £167, of which £51.40 went on a car.
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This car spending included £16.40 for fuel, £9.50 on insurance and £6.10 for repairs and servicing.
RAC Foundation director Professor Stephen Glaister said: "Before tax we have some of the cheapest petrol and diesel prices in Europe but when you add in fuel duty and VAT the picture changes dramatically."