The Mail on Sunday (MoS) falsely claimed that Labour was planning to scrap a tax exemption on homeowners, in a prominent story that has since been used by the Conservatives as part of their election campaign.

The press regulator Ipso ruled that the newspaper’s reporting was inaccurate and “could cause significant concern to readers that, under a Labour government, they could be liable to pay a tax they are exempt from under current legislation”.

The MoS headline was then used in Conservative campaign literature and shared by Tory politicians on social media.

The story said Labour wanted “to scrap the capital gains tax exemption on main homes”, citing a report called Land for the Many, written for Labour by a group of academics and campaigners.

Contrary to the newspaper’s claims, the third-party report explicitly rejected proposals to scrap the capital gains tax exemption for main homes, and instead recommended an annual levy on any increase in value of a property.

The erroneous article was published in June, and the press regulator ruled on the inaccuracy in November. The MoS must now publish Ipso’s ruling on page 2 of its print edition and on the top half of its website for 24 hours. But because the paper sought a review of the process by which the decision was made, publication of the correction has been delayed until after the election.

The authors of the Land for the Many report, which was edited by the Guardian columnist George Monbiot, have gone public with the ruling before Thursday’s vote.

Monbiot said: “We are pleased that Ipso has now imposed a major sanction. But I fear the damage has been done. The false claim has been implanted in people’s minds that Labour is coming for your home.”

The false claims were repeated on Conservative party campaign websites such as Cost of Corbyn, where the policy was described as a “movers tax”. They were also cited on the spoof Labour Manifesto website produced by the Tories, which was promoted heavily with paid-for online adverts.

The MoS argued that although the Land for the Many report did not propose scrapping the exemption for main homes, the article was not inaccurate as the report proposed an alternative tax on the gains in capital value of main homes.

However, Ipso’s complaints committee concluded that the MoS “inaccurately reported information featured clearly within a publicly accessible policy document”.

It said: “The correction offered by the newspaper was insufficient to address this significant inaccuracy, which had formed a central point in the article. In light of these considerations, the committee concluded that an adjudication was the appropriate remedy.”

Separately, the regulator confirmed on Monday that it had received a complaint regarding an article in the Sun claiming that Jeremy Corbyn was at the centre of an “extraordinary network of hard-left extremists”. The Sun quietly removed the article from its website after it was pointed out that the underlying research included information from far-right and conspiracy websites.

The story published on Saturday was based on a diagram originally called the “traitors chart”, which was put together by “a former British intelligence officer”. It linked the Labour leader to hundreds of individuals through his connections to organisations as diverse as the Stop the War Coalition and supposed leftwing factions in the British Medical Association.

A spokesperson for the Sun declined to comment on why the article, which carried the political editor Tom Newton Dunn’s byline, had been deleted from the website without acknowledgement. The online link now redirects to a page suggesting it was removed for legal reasons.

Peter Geoghegan, of the political website openDemocracy, which was listed as one of the points on the diagram, tweeted last week: “I’ve never complained to Ipso before. But I’m very tempted to now. Unless Tom Newton Dunn and the Sun retract this cheap smear immediately. Myself and my colleagues have been nominated for ‘investigation of year’ at 2019 British press awards and much more.”