Tory Justice Secretary Robert Buckland has claimed there is a "sinister" campaign to encourage tactical voting against Tory candidates.
He claimed to have seen leaflets urging people to back candidates tactically which did not include an "imprint" - a legal note telling the reader who produced the ad.
And he warned the Tories could "look again" at campaigning rules after the election, telling LBC Radio: "After this election we need to just consider again how the rules are policed."
It comes after a furore over page 48 of the Tory manifesto, which hints at sweeping reforms to the entire legal and governing framework of the UK.
It says: "After Brexit we also need to look at the broader aspects of our constitution: the relationship between the Government, Parliament and the courts; the functioning of the Royal Prerogative; the role of the House of Lords; and access to justice for ordinary people."
Mr Buckland spoke on LBC Radio. He added: "There's an orchestrated campaign going on in some seats, so-called tactical voting and very negative, unattributed literature going out about Conservative candidates."
"No imprint on them," Mr Buckland said. "No way of working out who is publishing it. This is frankly a little bit sinister."
Mr Buckland told LBC he had "evidence of leaflets listing things about Conservative candidates what might be considered as negative", though he did not go into detail.
"I think some of it very much hits the border".
Speaking of the tactical voting leaflets he said: "That is what I am talking about, is this sort of underhand, cloak-and-dagger third party [activity].
He added: "My advice to everybody is play within the bounds of the law. Be respectful. Do not use this general election as an excuse to create some sort of agitation in a very negative way. Let’s be positive about our campaigns."
His comments came as, separately, campaign group Our Future, Our Choice called for Labour and Lib Dem candidates in seats where they have a slim share of the vote to endorse the candidate most likely to beat the Tory candidate.
Co-founder Femi Oluwole suggested a 'candidate endorsement swap' deal, where 10 Labour and 10 Lib Dem candidates to 'pair up'.
Mr Oluwole said: “Many candidates are standing in seats where the 2017 result and current polling show they have no chance of winning.
He added: "All they’ll do is take votes from the candidate most likely to beat the Tory and let the Tories win. That’s the picture we are seeing around the country.’
“These candidates need to endorse the candidates most likely to beat the Tories in their area.”
