One-fifth of Americans have said they believe Barack Obama, the US president, is a Muslim, according to a new opinion poll.

Time poll, published on Thursday, also suggested that almost a third of Americans said Muslims should be barred from running for president or serving on the US Supreme Court.

Bill Burton, the White House deputy press secretary, said hours after the poll was published "the president is obviously a Christian. He prays everyday".

Americans care more about the economy and wars in Iraq and Afghanistan and "are not reading a lot of news about what religion the president is", he added.

Another poll conducted by Pew Forum on Religion and Public Life showed that 18 per cent of Americans said that Obama is a Muslim.

Pew poll results are based on interviews conducted before the controversy over whether Muslims should be permitted to construct a centre near the World Trade Center site in New York.

Obama affirmed  last week the right to build the centre on religious freedom groundsbut, saying he will not take a stand on  the "wisdom" of doing so.

Time conducted its survey afterwards.

Muslim centre controversy

Time poll showed that 61 per cent of Americans opposed building the centre near the Trade Center site, where the September 11 attacks took place, while 26 per cent said they favour it.

The Pew study noted the overall number of those saying Obama is a Muslim had spiked from 11 per cent in March 2009 but that the view "is more widespread among his political opponents than among his backers".

Still, "even among many of his supporters and allies, less than half now say Obama is a Christian. Among Democrats, for instance, 46 per cent say Obama is a Christian, down from 55 per cent in March 2009", Pew said.

Some believe that, with congressional elections just over two months away, the polls may represent bad news for the Democratic party.

Pew analysts attribute the findings to attacks by Obama's opponents and Obama's limited attendance at religious services, particularly in contrast with Presidents George W. Bush and Bill Clinton, whose worship was more public.

Andrew Kohut, Pew Forum's director, said the confusion partly reflects "the intensification of negative views about Obama among his critics".

'24-hour noise box'

Pastor Kirbyjon Caldwell of Houston told the Associated Press news agency he knew Obama for years and said the president is a Christian who prays every day.

He called false media reports about it "a 24-hour noise box committed to presenting the president in a false light."

Six in 10 of those saying Obama is a Muslim said they got the information from the media, with the largest portion, 16 per cent, saying it was on television.

Eleven per cent said they learned it from Obama's behaviour and words.