The X-Men star is horrified by proposed government budget cuts which he claims will have a detrimental effect on the country's cultural institutions.
He spoke out about the cutbacks last year (10) and now he's ramped up his campaign by visiting the prime minister's London headquarters to deliver a letter from performers' union Equity.
Stewart also took the opportunity to call for a summit at Downing Street to address the arts community's concerns.
He says, ""We are requesting (ministers) begin to formulate a serious, significant and ongoing policy for arts. No Conservative government has ever been strong on arts policy. This coalition (government) is no different.
""The arts should not have been cut at all. It is one of the ongoing success stories of the UK. Our ballets, operas, theatres and so on are the reason 80 per cent of people come to the UK. We're a great money-earner. Cutting back on funding makes no sense.
""We're here to convene a meeting between arts organisations, Government and artists to determine what is the coalition's arts policy. There doesn't seem to be one. When we hopefully come out of recession, are they going to reinstate funding? The damage will be endless.""
Cameron was not actually present at Downing Street at the time of Stewart's visit - he is currently on holiday in Spain with his wife, Samantha.