It is a test that all Indian and other non-EU citizens have to pass before becoming British citizens, but Prime Minister David Cameron last night had a tough time answering some key questions on a widely watched American chat show.

Cameron, who is an Oxford graduate, appeared on the Late Show with David Letterman in what was considered by some in London as a risky move, given the host's biting humour and obvious relish in embarrassing high profile guests.

Cameron, who announced in a major speech on immigration in October 2011 that his government was revising the citizenship test to "put British history and culture at the heart of it", could not answer questions about the Magna Carta and the composer of Rule Britannia.