A Greater Manchester Police spokesman confirmed that a report was received and “we will be investigating this”.
A statement from the Electoral Commission said: “All printed material promoting a candidate at an election is required to include an imprint identifying the promoter and printer of the material, so voters know who is responsible for it.
“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 spokesman for Hardings Print Solutions said: “We take our responsibilities in relation to election material very seriously and take full responsibility for this error occurring.
“Reform UK did not request or authorise the removal of the imprint.
“The omission arose from Hardings Printers’ production process.”
The by-election was triggered by former Labour Andrew Gwynne standing down on medical grounds.
It will be held on Thursday 26 February, with the result expected to be announced in the early hours of Friday morning.
GB News presenter and academic turned activist Matt Goodwin has been chosen as the Reform candidate.
A full list of candidates for Gorton and Denton can be found here.