The Association of Greater Manchester Authorities' proposed bylaw would see stores in the area forced to charge at least £4.50 for a 75cl bottle of wine and £6 for a six-pack of lager. However, it could not be enacted without the support of the Home Secretary.
Cameron said: "The idea of the councils coming together on this is a good one. Where there can be local decisions we are happy for that to happen."
Association of Convenience Stores chief executive James Lowman called the PM's support for the Manchester scheme "irresponsible".
"If local minimum pricing is implemented, local shops and national businesses alike will be hit hard by people shopping elsewhere so that they can avoid the restrictions," he said.
The plan is likely to face a legal challenge from retailers and suppliers, who believe it breaches European competition law.
The Prime Minister did offer some hope for stores struggling to compete with supermarket price-slashing. "Stopping supermarkets from selling 20 tins of Stella for a fiver, that's what we've got to go after. Where I want to help is ending the deep discounting on alcohol."