Image caption Richard Grenell previously worked as a spokesman for the UN mission under President George W Bush

A gay spokesman for presidential candidate Mitt Romney has quit on his first official day of work, amid criticism by anti-gay conservatives.

Richard Grenell, recently hired to speak on foreign affairs for the presumptive Republican nominee, announced his departure on Tuesday.

In a statement, Mr Grenell thanked Mr Romney for "his belief in me".

The spokesman had previously deleted about 800 tweets and took down his personal website.

According to the Washington Post Mr Grenell had come under fire for statements about Callista Gingrich and Michelle Obama.

The Romney campaign said it was "disappointed" that Mr Grenell had resigned.

"We wanted him to stay because he had superior qualifications for the position he was hired to fill," Romney campaign manager Matt Rhoades said in a statement.

'Drop dead'

Mr Grenell was criticised quickly after he was hired.

Bryan Fischer, director of issues analysis for the American Family Association, a group opposed to homosexuality, wrote a blog post on 20 April attacking Mr Romney's choice, saying it sent a "message to the pro-family community: drop dead".

Another writer in the conservative publication National Review argued that Mr Grenell's apparent obsession with gay marriage could damage the Romney campaign.

Contributor Matthew Franck suggested that the spokesman would switch to the Obama campaign if the president included support for same-sex marriage in his convention speech.

In a statement, Mr Grenell cited "personal reasons" for his decision to leave.

LGBT groups and surrogates for Mr Obama's campaign have argued he was forced out because of his sexual orientation.

"This is the kind of bigoted, anti-gay extremists a Romney administration would find itself held hostage to," pro-Obama super PAC founder Bill Burton told the Post.

The director of the gay Republican group Log Cabin Republicans said the spokesman had been "essentially hounded by the far-right and far-left".

"It is unfortunate that while the Romney campaign made it clear that Grenell being an openly gay man was a non-issue for the governor and his team," R Clarke Cooper said, "the hyper-partisan discussion of issues unrelated to Ric's national security qualifications threatened to compromise his effectiveness on the campaign trail."