The Conservatives and Liberal Democrats have denied sending out an anonymous leaflet in a key north London constituency that urges voters to oust the Labour incumbent by voting tactically.

The local Labour party has made a police complaint about the mysterious flyer calling on residents to vote Lib Dem in Islington South and Finsbury, where the sitting MP is Labour’s Emily Thornberry.

Electoral law means that material which “can reasonably be regarded as intended to influence voters to vote for or against a political party” must bear the name and address of the publisher to ensure transparency.

The flyer distributed in Islington South and Finsbury. Photograph: The Guardian

The flyer, which appeared in letterboxes over the weekend, is edged in Tory blue and gives vote tallies for the 2005 and 2010 elections to illustrate how a vote for the Lib Dems by Conservatives could deny Labour the seat. It is 36th on the Lib Dems’ list of target seats.

Thornberry had a majority of 3,569 in 2010 – a significant increase from 484 in 2005. She said she had no idea who could be behind the leaflet and refused to speculate on a potential funder.

“We’re calling it what it is, it’s a US-style dirty tricks campaign,” she said. “It’s illegal, it’s unethical and we’re cooperating fully with the police.”

Thornberry, who was forced to resign her shadow cabinet position in November over a tweet featuring a St George’s flag, said she had seen the leaflet in a couple of wards over the past 24 hours, adding that the local party would keep an eye out for more.

A Conservative party spokesman said the leaflet had “nothing to do with us”. Helen Roberts, the Islington Conservatives election agent for local candidate Mark Lim, said she had reported it to Islington’s electoral services team.

The Islington Lib Dems’ spokesman, James Williamson, also denied any connection, calling the flyer “stupid” and “illegal”.

The Lib Dem candidate Terry Stacy tweeted that the leaflet was not his, but that he did not disagree with its content.

@TomJSpencer @IslingtonLibDem Nothing to do with us Mr Spencer, but must agree, only way to beat Lab in Islington South is to vote Lib Dem — Terry Stacy MBE JP (@TerryStacyLD) May 5, 2015

Gill Barr, a resident, received the leaflet at the weekend and said she thought it was from a political party. “I find it very weird,” she said, “and that’s got nothing to do with my own political preference. I do think it’s hard to have respect for anyone who won’t put their name to a leaflet like this, and according to the Electoral Commission, it’s illegal.”

Retired BBC journalist Martin Shankleman also received one of the leaflets and told the Guardian he found it difficult to believe it was not linked to a party. “Lib Dems are the prime suspect, as they would benefit from the tactical vote,” Shankleman said. “They have a reputation for fighting mean.”

An Electoral Commission spokesperson said: “The law [Political Parties, Elections and Referendums Act 2000] states that all printed material [ie posters, placards and leaflets] must include the name and address of the printer and promoter [the person who authorised the material to be printed]. This is so that electors can be clear about the source of the campaign material.

“It is an offence not to include an imprint on election material and any breach of the rules will be considered in line with the Electoral Commission’s enforcement policy.”

The Metropolitan police said in a statement: “We have today Tuesday, 5 May, been made aware by the media of an election leaflet being circulated in Islington that is said to have no imprint, and, as such, be in breach of the Representation of People Act, 1983. Detectives from the Special Enquiry Team are now investigating this allegation.”