The European Union's president is warning Britain that he's open to suggestions on how the bloc must change, but that "the fundamental values of the European Union are not for sale and so are non-negotiable."

EU President Donald Tusk spoke Thursday as bloc leaders gathered for a summit in Brussels.

British Prime Minister David Cameron said he'll use the two-day event to explain to other heads of state and government that the 28-nation EU must reform or risk losing the backing of a majority of British voters.

Cameron and his Conservative Party won a general election in May. Cameron has promised a referendum by 2017 on whether Britain, an EU member since 1973, stays in the bloc or not.