Charities and voluntary groups would be able to host functions at grace-and-favour residences under a new Tory proposal.
Francis Maude, the shadow Cabinet Office minister, said the Conservatives wanted to open the empty or little-used mansions Dorneywood in Buckinghamshire, Chevening in Kent and 1 Carlton Gardens in London for receptions and events for good causes rather than reserving them exclusively for ministerial use.
"Properties bequeathed to the nation will once again benefit the nation," he said.
Liz Atkins, director of public policy at the NCVO, said it would be "only a good thing" if more ministerial buildings were opened up to charities. She said the move might boost public trust in politicians after the row over MPs' expenses.
Chevening is an official residence for the Foreign Secretary, as is 1 Carlton Gardens in London, which has been empty since 2007.