George Galloway has denied claims made by Jemima Khan in the New Statesman that he converted to Islam in a ceremony in London more than 10 years ago.

The newly elected MP for Bradford West does not deny being a Muslim, but says Khan's claim of a conversion in a hotel in Kilburn, north-west London, is "simply and categorically untrue".

Galloway is often asked about his faith but refuses to answer, saying his religion is a "personal matter" of no import to his political activities. He recently married his fourth wife in what has been reported was a Muslim ceremony in Amsterdam.

In a statement entitled "The Staggers has fallen on its face", Galloway responded to the New Statesman piece by saying: "The opening paragraph of Jemima Khan's piece in the New Statesman [referring to an alleged conversion ceremony] is totally untrue. Moreover, I told her it was fallacious when she put it to me. I have never attended any such ceremony in Kilburn, Karachi or Kathmandu. It is simply and categorically untrue."

Khan's piece starts with the claim that Galloway converted at a ceremony attended by members of the Muslim Association of Britain. "Those close to him know this. The rest of the world, including his Muslim constituents, does not," writes Khan. She goes on to describe Galloway's reaction when she told him someone she knew also attended the "shahadah" conversion ceremony, claiming he did not respond, instead staring at her across the table and then getting up to leave the interview.

On Thursday, a spokesman for the Muslim Association of Britain said he had not heard of any such ceremony taking place in the presence of his members. Mohamed Kozbar, from the north London branch, said: "I've never heard anything about that. I don't know if George Galloway is a Muslim, but he is supportive of the Muslim community."

In his rebuttal, Galloway accuses Khan of reporting "deliberate falsehoods" and making "schoolgirl howlers which would earn banishment from a first-year journalism class".

He said: "For instance, she misspells the name of my 'glossy haired' secretary, who is not my special assistant. Snidely, she claims that I have a slow and over-enunciated delivery – even then she failed to pick up the facts! – and, absurdly, says that's because I have a Glasgow accent which would require subtitles for those for whom English isn't a first language. Putting aside the regional and racial slurs and the cloth ear for accents, I'm from Dundee! Which most of the rest of the world knows, but certainly the entire British press corps. I could go on."

Galloway is demanding a retraction from the New Statesman.

The New Statesman said in a statement: "It is notable that Galloway does not deny being a Muslim convert – and he did not deny it when it was put to him at the time of the interview, which is on tape. Contrary to his press release, nor did he deny that the ceremony took place when it was put to him during the interview. This is also on tape. Furthermore, he failed to clarify how, by his own admission, he had a "nikah" (a Muslim marriage ceremony), despite the fact that a non-Muslim man cannot marry a Muslim woman under Islamic law. As for calling his "secretary" his "special adviser", this is how she asked to be described in an email to Jemima Khan."

Khan added: "As for his Glasgow accent, he was MP for the city for 18 years, but if he disagrees that he picked up any inflection while there, we'll happily run a letter to that effect."