"I don't see why not [allow ministers to campaign]. It would be safer and more harmonious to say 'make your own minds up,'" Johnson said during his weekly Leading Britain's Conversation (LBC) radio show.
He cited a 1975 UK referendum to assess public support for UK membership of the European Economic Community, currently integrated into the European Union, as a good example of when ministers were free to vote according to their conscience.
On Tuesday, the UK parliament is debating the issue of a referendum on UK membership of the European Union that, Cameron pledged, will take place before the end of 2017.
Before winning the May 7 general election, the UK prime minister vowed to hold a referendum on the country's EU membership and revise Britain's relations with the union, should his party emerge victorious.