A think-tank is warning that free hospital parking would see less money raised for front-line health services.
It follows a campaign by the Conservative MP for Harlow, Robert Halfon, who is renewing his call for charges to be scrapped.
Mr Halfon has described them as a stealth tax on the sick.
However, the Adam Smith Institute, which aims to promote the economic idea of free markets, says the removal of charges would also lead to drivers abusing the system.
But Mr Halfon says there are ways around that.
He said: "You'd have a token or ticketing system. There are already barriers in hospital car parks so you could get the token either because you're a sick patient or visiting."