Almost one in five Americans believe that Barack Obama is a Muslim, according to a new poll, despite his public statements about his Christian faith.

The survey by the Pew Research Centre found that 18% now say that the US president is a Muslim, up from 11% in March 2009. Only about one-third of adults (34%) say Obama is a Christian, down sharply from 48% in 2009. Forty three percent say they do not know Obama's religion.

The survey was completed in early August, before Obama's recent comments about the proposed construction of a mosque near the site of the former World Trade Centre, which have landed him in political hot water. The president has said he believes Muslims have the right to build an Islamic centre there, but refrained from taking a position on whether or not it should actually be built two blocks from Ground Zero. The issue has become politically charged ahead of congressional races in mid-November, with Republicans accusing Obama of being out touch with mainstream America.

The Pew Research Centre noted that the belief that Obama is a Muslim has increased most sharply among Republicans, (up 14 points) since 2009, especially conservative Republicans (up 16 points). But the number of independents who say Obama is a Muslim has also increased significantly (up eight points). There has been little change in the number of Democrats who say Obama is a Muslim, but fewer Democrats today say he is a Christian.

The White House is concerned that beliefs about Obama's religion are linked to political judgments about him. Those who say he is a Muslim overwhelmingly disapprove of his job performance, while a majority of those who think he is a Christian approve of the job Obama is doing. Those who are unsure about Obama's religion are about evenly divided in their views of his performance.

The White House blamed "misinformation campaigns" by the president's opponents.

"While the president has been diligent and personally committed to his own Christian faith, there's certainly folks who are intent on spreading falsehoods about the president and his values and beliefs," Obama's faith adviser, Joshua DuBois, told the Washington Post.

DuBois pointed to six speeches on faith that the president has given in which he talked about his beliefs, but said coverage of Obama's Christianity has been scant compared with news about the economic crisis, legislative battles and other issues.

Pew analysts attribute the findings to attacks by his opponents and Obama's limited attendance at religious services, particularly in contrast with presidents George Bush and Bill Clinton. Andrew Kohut, the Pew Research Centre's director, said the confusion partly reflected "the intensification of negative views about Obama among his critics". Alan Cooperman, the Pew Forum's associate director for research, said that with the public hearing little about Obama's religion, "maybe there's more possibility for other people to make suggestions that the president is this or he's really that or he's really a Muslim".

Obama is the Christian son of a Kenyan, Barack Obama Sr. Obama Sr was raised Muslim.

Between the ages of six and 10, Obama lived in predominantly Muslim Indonesia with his mother and Indonesian stepfather.

During the election campaign, rightwing commentators such as Rush Limbaugh used his full name, Barack Hussein Obama, in an attempt to portray him as somehow un-American. Rightwingers also spread the false allegation that Obama had attended a madrassa in Indonesia.