Former President Laszlo Solyom - who helped to draft the 1989 constitution and served as the head of the Hungarian Constitutional Court from 1990 to 1998 - said in an open letter to the press on the day of the vote that the only way to stop the changes becoming irrevocable would be for President Janos Ader to refuse to sign the constitutional amendments into law.

BBC: Q&A: Hungary's controversial constitutional changes