Daniel Radcliffe has announced that he is no longer a supporter of the Liberal Democrats after emerging as one of the party's most high-profile celebrity backers ahead of the last British general election, and will probably vote instead for Labour under its "genuinely leftwing" leader, Ed Miliband.

In what is turning into a hemorrhaging of support for the Lib Dems among a list of celebrity backers it unveiled in the run-up to last year's vote, the star of the Harry Potter franchise described party leader Nick Clegg as a "whipping boy" for the Conservatives. He also hit out at the "homophobia" of some of the US Republican presidential candidates.

Colin Firth, another actor and A-list Hollywood star declared in December that he was ending his support for the Lib Dems. The party has also lost the support of Bella Freud, the fashion designer, and Kate Mosse, the author.

Radcliffe made the comments in an interview that will be published on Monday in the latest issue of Attitude magazine, the same forum he used in 2009 to announce that he would "almost certainly" be using his first ever vote in a general election to vote Lib Dem.

Asked if he is happy with the Lib Dems's place in the coalition, he said: "No, of course not. Nick Clegg asked to meet me after that Attitude interview and we talked about issues such as gay rights and faith schools.

"I was initially supportive. For me it was good that the Lib Dems would be fighting our corner. But he has become a whipping boy and it seems to me that he has been totally used by the Tories - anything they don't want badly reflected on them they reflect on to him."

The actor, who is estimated to have a £30m fortune, cited "so many concessions" by the Lib Dems' on education and taxes. He added: "I think, if you make a lot more money than most people - like I do - you should pay more tax and subsidise people who work just as hard as you, but don't earn as much."

Radcliffe, whose current film, The Woman in Black, was estimated to have made $21m at the US box office during its weekend opening, said he "will probably be going to Labour".

He said: "From what I've seen of Ed Miliband, I really like him and he speaks for what I believe in. I think he's genuine, genuinely leftwing, and will act as such if he gets in."

The actor — who is straight — also used the interview to call for gay marriage, relationship education in schools that would cover both gay and straight relationships, and attacked some of the US Republican presidential candidates.

Radcliffe said that he wished more educational establishments, especially in the US, were not in thrall to religion, stating: "I'm not religious, I'm an atheist, and a militant atheist when religion starts impacting on legislation. We need sex education in schools.

He went on to say that he has been "disgusted, amazed, stunned" by candidates seeking the Republican presidential nomination, such as Rick Santorum or Michele Bachmann, who have been openly hostile to gay rights.

"But they disgusted me less than candidates like Rick Perry, who made that ridiculous advert wearing 'the Brokeback jacket', and I think pretend to be homophobic just to win votes." .

Asked if he wished that Barack Obama would publicly back gay marriage, he replied: "Yes, I do, but can he really? Of course he's in favour of it, but he has to be careful about saying so. I'd rather have someone like him in the White House than the alternative."