Reform UK will face a police investigation in Gorton and Denton after admitting that it sent out letters from a “concerned neighbour” which did not state that they had been distributed and funded by the party.

Greater Manchester police confirmed it had received a report about the breach of electoral law and said it would investigate. The Electoral Commission said the omission was a matter for the police, stressing that failing “to include an imprint in candidate election material is an offence.”

Dozens of voters in Gorton and Denton constituency reported receiving letters from a pensioner written in a handwriting-style printed font on Friday. The letters do not contain an imprint saying who they have been funded and distributed by, as required by electoral law.

The author of the letter, Patricia Clegg, describes herself in the mail outs only as a “local pensioner, 74 years old”.

When contacted by the Guardian, Clegg confirmed she was a member of Reform UK and had been asked to write the letter by the party.

“I was asked to support reform, would I be willing to do a letter and put my name to it? And I said, Yeah, and I left the rest to them,” she said when asked if she had written the letter.

Asked if she was aware that the letter did not have the legally necessary imprint saying it was backed by the party, she said no one had talked to her about that issue.

A Reform spokesperson said the campaign had commissioned the letter which was given to its print contractor “with the full and correct legal imprint, fully compliant with election law” but “an error occurred during the printing process”. Subsequently the legal imprint was “inadvertently removed at the point of printing” without the party’s knowledge.

“At no stage did the campaign know about, authorise or intend the distribution of material without a legal imprint.,” said a spokesperson. Hardings Print Solutions Ltd in Middlesex, which printed the letter, said it “took full responsibility” for the error.

A spokesperson for Greater Manchester police confirmed that it had received a report about the breach of electoral law on Friday evening. “We will be investigating this,” they said.

The Electoral Commission said all printed material promoting a candidate at an election is required to include an imprint identifying the promoter and printer. “Failing to include an imprint in candidate election material is an offence. Enforcement of imprint requirements for candidate material is a matter for the police,” a spokesperson said.

Dozens of voters reported receiving the letter, which stated in printed letter-writing script that the author had previously “voted Labour because Keir Starmer told us things would change for the better. They haven’t.” It added that tax rises “have cost pensioners like me an extra £160 that we cannot afford”.

The author also says they “understand why some neighbours have had enough of Keir Starmer and are thinking of voting Green”.

“But I do not believe the Greens have answers to our problems,” the letter says. “They have extreme policies like legalising drugs and letting men use women’s changing rooms. What good would that do people like us?”

Several copies for the letter were posted on local Facebook and WhatsApp groups, with people saying they received them on Friday morning.

One voter, who said they are not affiliated to any party, said they had reported the matter to the Manchester city council’s elections office.

The byelection is expected to be tightly fought between Labour, the Greens and Reform, with all three claiming they are on course to win.

On Friday a spokesperson for the Green party accused Reform of “playing dirty”.

“With Reform raking in millions from crypto billionaires and fossil fuel giants, you’d think they’d be able to afford to follow the law,” they said. “But instead they’re playing dirty — because they know Greens are coming for them in Gorton and Denton.

“The only way to stop Reform’s politics of division and hate is to vote for Hannah Spencer – to elect a Green MP who will fight to lower bills, protect the NHS and rebuild our public services.”

Labour’s campaign political lead Andrew Western MP said: “Campaigns are responsible for what they put through people’s doors. Blaming a printer doesn’t remove that responsibility or excuse the failure to meet basic legal requirements. Reform should concentrate on getting the basics right.”