Lib Dems hope Potter will cast spell on voters

Actor Colin Firth, scientist and author Professor Richard Dawkins, satirist Armando Iannucci and human rights and environmental campaigner Bianca Jagger were also on the list released yesterday.

Lib Dem leader Nick Clegg said: "I welcome the support of everyone who recognises this is a once-in-a-lifetime opportunity to make a real difference in Britain."

Professor Dawkins said he was backing the party because of their commitment to scrapping the "ludicrous" 'first past the post' voting system, while Firth, who won a Bafta for his role in A Single Man, said: "As a once-committed Labour voter I, like so many, have been appalled by the abandonment of the values they advocated while in opposition. For me, their conduct on asylum alone is reason enough never to be able to contemplate voting for them again."

Mr Clegg accused David Cameron of "breathtaking arrogance" as he accused the Tory leader of already "measuring up the curtains" of 10 Downing Street before votes were cast.