Labour’s deputy leader has said an investigation will be launched into who made a “completely unacceptable” claim anti-Semitism among Muslim voters was costing the party votes.

The Mail on Sunday quoted a “senior Labour official” as having said Labour was “haemorrhaging votes among Muslim voters” in the run-up to the Batley and Spen by-election on July 1.

“The reason for that is what Keir [Starmer] has been doing on anti-Semitism.” the source told the newspaper.

“Nobody really wants to talk about it, but that’s the main factor. He challenged [Jeremy] Corbyn on it, and there’s been a backlash among certain sections of the community.”

Angela Rayner said in response: “I want to make clear publicly that these comments that are being attributed to a member of Labour Party staff in a newspaper today are not a Labour Party response or statement, are completely unacceptable and are not condoned or sanctioned in any way by the party.

“Anybody who has made these comments should and will be dealt with in line with our independent disciplinary procedures, which I have no role in as deputy leader.”

The Labour Muslim Network also condemned the “patently vile, Islamophobic briefing”.

“This racism needs to be challenged urgently and publicly by the Labour leadership and the party as a whole,” the group said.

“There can be no hiding behind the anonymity of the source and briefing.”