One of the golden rules of journalism is that you don’t break an embargo – a block on publication until a certain date. For the first time in my career, I’m breaking one now. The moral case for doing so outweighs the moral case for respecting it.

For five months the press regulator, Ipso, which was set up by British newspapers, has been considering a complaint that could play a role in this election. It concerns an outrageous lie about the Labour party first published by the Mail on Sunday and repeated many times by other newspapers and the Conservative party.

Two weeks ago, Ipso ruled against the paper, and imposed a major sanction. The Mail on Sunday asked for a review of the decision, delaying its publication.

As far as voters are concerned, the lie stands. A key claim used to tip the balance against Labour remains uncorrected

The review upheld the decision, but it still hasn’t been published. So, as far as voters are concerned, the lie stands. A key claim used to tip the balance against Labour remains uncorrected. I owe more to the truth than I do to the embargo.

Last summer, I co-published a report to the Labour party titled Land for the Many. It took many months of work, drawing on a vast range of sources and expertise, to produce radical but realistic ideas for changing the way we use our most important resource. We worked hard to make our arguments as watertight as possible. We needn’t have bothered, because our opponents scarcely addressed them.

The storm of lies about our report in the billionaire press gave me an idea of what it must be like to be a Labour politician. Pushing back against them all would be a fulltime job for several people. But we decided to confront one of them in the hope that it might illuminate the others. The Mail on Sunday turned our independent report for the Labour party into “bombshell plans being drawn up by Labour leader Jeremy Corbyn”, then told an outright lie about what it contained.

We proposed, it said, “to scrap the capital gains tax exemption on main homes”. At the moment, if you sell the house or flat in which you live, you don’t pay capital gains tax on the profit you might make. We examined the case for changing this, explained the arguments for and against, then clearly and specifically rejected the idea. But who cares, if you can terrify the living daylights out of your readership, convincing them that Corbyn is the spawn of Satan?

As if to suggest that such lies are organised with the Conservative party, the article quoted Boris Johnson, who claimed “this mad ‘tax on all your houses’ would cripple every Brit who owns or wants to own their own home”. The lie was picked up on social media by other senior Conservatives, and has been used repeatedly in the party’s campaign materials, websites and Facebook pages. It was reproduced by most of the other billionaire papers, and continues to be circulated. Just last week, an article by Tom Newton Dunn in the Sun claimed that Labour was considering a “movers tax”, “scrapping the capital gains exemption on main homes”. The false claim has now been deeply implanted in people’s minds: Labour is coming for your home.

One of the authors of our report, Anna Powell-Smith, made a complaint to Ipso. Had she not been determined, she might have been defeated by a process that seems designed to deter. For much of the five months this has taken to resolve, the newspaper, with its vastly greater resources, was allowed to bombard her with Johnsonian arguments, or offer tiny clarifications on a remote page. It was time-consuming and intimidating. Most people are likely to have given up or accepted a meaningless concession. Watching this process, I came to the conclusion that Ipso is not fit for purpose.

Even so, it eventually bowed to the inevitable. This is a rare victory against the billionaire press, but it would count for nothing if buried until the election is over. Anyone who wants a better world finds themselves at war with the exceedingly rich people who own the media and the editors and journalists they employ. The pen might be mightier than the sword, but the wallet is mightier than the pen. News is the propaganda of the oligarch. Are we prepared to allow the proprietors of the newspapers, many of whom live offshore, to determine the course of our politics?

Mail on Sunday made false claims about Labour's tax plans Read more

The rewards for political lying are massive: they include winning referendums and elections. The penalties are either nonexistent or tiny. If the Conservative party is sanctioned by the Electoral Commission for any of its outrageous lying and cheating, it might, long after the election, have to pay the maximum fine of £20,000, which a friendly billionaire could doubtless pull out of his back pocket.

If elections are won by lies, we find ourselves governed by liars. They won’t hesitate to ramp up their deceptions when in office. One focus is likely to be voter suppression. Already the Conservatives, learning from Republican tricks in the US, have floated the idea that people must bring photo ID to the polling booths. They know that 3.5 million people, few of whom are likely to vote Conservative, don’t possess it.

The old adages – cheats never prosper, the truth will out, virtue always triumphs – are themselves falsehoods. But, futile as it often seems, one-sided as the war between truth and falsehood always is, we must fight the tide of lies. Don’t let them win this week.

• George Monbiot is a Guardian columnist