Chairman of the Australian Monarchists League, Philip Benwell, accused the South Australian beer company Coopers of attacking Australia's constitutional monarchy.
"It crossed a boundary, because it said 'Forget the Monarchy', and that is a political statement. Why not say forget the republicans?" he told Reuters.
Coopers appeared to have backed Prime Minister Kevin Rudd's support for a constitutional shift cutting ties to Britain's monarchy, replacing the Queen as the country's head of state with a presidency, Mr Benwell said.
Mr Rudd ended a visit to Britain in April with an audience with Queen Elizabeth II and a prediction that debate on a republic would grow in Australia in 2009, although for the moment his Government had "bigger fish to fry".
A 1999 referendum vote engineered by former conservative prime minister and monarchist John Howard was defeated after republicans were unable to decide what type of presidency the country should have, despite wide public support for a shift.
"We are on the watch for these types of things. It could be said to be humorous, but we felt it was more political than humorous, and that's something that we have to protest," Mr Benwell said, adding monarchists were bracing for another referendum.
Coopers withdrew billboards featuring the ad after complaints to Australia's Electoral Commission and Advertising Standards Board by the League, whose website says it supports "Australians protecting the Constitution".
The brewer, established in 1862, is Australia's last family owned brewery, and produces ales, stouts and lagers sold across the country and internationally.
Communications and advertising consultant Jane Caro said the monarchist complaint could backfire given irreverent Aussie humour and fondness for beer.
"It's a triumph of pomposity. I think the monarchists should apologise to us all for making the world a little duller," Ms Caro said.


