A leaflet attacking Labour Friends of Israel and attributed to George Galloway's Respect Party has been distributed ahead of next week's Rotherham by-election.

The vote was called after the longstanding Labour MP stepped down earlier this month in the wake of a scandal over his parliamentary expenses. The Respect party is hoping to replace Labour's Denis MacShane with its candidate Yvonne Ridley, a Press TV presenter.

In a leaflet circulated this week in Respect's name, and posted on the Rotherham by-election blog, Muslim and Asian voters are urged to "send a message to the sickening racism, Islamophobia and corruption" of Labour.

The ex-MP is referred to as "MacSHAME", and attacked for having "set up an Israeli support group the so-called Labour Friends of Israel…at a time as our Muslim brothers and sisters in Palestine were being cut to pieces and burned alive by Israeli bombs".

Mr MacShane did not set up LFI, which was founded in 1956, although he served as chair of the inquiry panel of the All-Party Parliamentary Group against Antisemitism, and was a member of the group.

The leaflet also accuses Labour of being a "party of closet racists and a party that traffics in anti-Islamic bigotry" and compares it to the National Front in the 1970s.

Ms Ridley denied that the leaflet had been distributed by the party or her own campaign. She told the Rotherham Advertiser that the leaflet was attempting to portray Respect as an Islamic party, "which it is not".

David Blunkett, the former Labour Home Secretary, told the Guardian that the leaflet was a disgrace and said Mr Galloway and Ms Ridley should "withdraw these claims or face action by the Labour Party, including taking this up with the Electoral Commission".

A spokesman for the Electoral Commission said it would not be investigating the matter because tit did not have any regulatory role in relation to the content of campaign material.

She added: "It is a criminal offence to make a false statement about the personal character of a candidate in order to affect the election of that candidate. Rules about defamation also apply to election materials."

Ms Ridley is an outspoken and frequent critic of Israel, writing on Twitter last week: "Why is Labour silent on #Gaza? #Respect supports the #Palestinian people and people in #Rotherham support them as well." Bookmaker William Hill have given her odds of eight to one to take the seat, putting Labour's candidate, Sarah Champion, at one to 20.