Democracy relies on accurate information. If voters are subjected to a cacophony of political claims between which it is almost impossible to sift fact from fiction, how can they be said to have made a meaningful choice? This is why the proliferation of “fake news” – false, fabricated or deliberately misleading information – poses such an existential threat to the legitimacy of the democratic process.

This election campaign is highlighting the extent to which it is not just shadowy players in the background who are involved in spreading fake news. It is increasingly something that certain politicians, in particular, the Tory party under Boris Johnson, are themselves creating and propagating in an effort to discredit their rivals.

It would be naive to imagine politics has ever been immune to spin and exaggeration. But until a few years ago, there was an assumption that most of what our leaders said would have some basis in fact and if a politician were caught telling a bare-faced lie, there would be political consequences. This helped to keep politics honest far more than any rule or law.

Many more will have seen the figures than will have read why they are utterly misleading

Last week’s campaigning shows how much this honour code has broken down. The Conservatives released a dossier that claimed to show a Labour government would increase spending by £1.2tn over five years. That figure has been roundly debunked by independent fact-checkers. Not only has Labour not yet published its manifesto, rendering any attempt to accurately cost what its plans might be completely invalid, but the Tory analysis includes policies Labour has explicitly said will not be in its manifesto and many that are not its policy. And some figures have been exaggerated through double-counting. This has not, however, stopped the Tories claiming that a Labour government would cost each taxpayer on average £2,400 a year. These figures have no basis in fact, yet Sajid Javid, the man who is asking voters for a five-year mandate to run the nation’s finances, robustly defended them.

It has been more of the same scaremongering about immigration. Priti Patel, the home secretary, claimed last week that net migration could treble to 840,000 people a year under a Labour government. Read the small print and not only have the Conservatives failed to set out their workings, the figure is based on the assumption that Labour would extend free movement rights to the whole world. This is not Labour policy.

Both figures have been comprehensively demolished. But in a world where many voters consume the news by scrolling through headlines on social media feeds, many more will have seen the figures than will have read why they are utterly misleading.

These tactics borrow heavily from the Vote Leave campaign of 2016, which was run by Johnson’s chief strategist, Dominic Cummings. Vote Leave deliberately misled voters in order to win votes. It produced a poster claiming “Turkey, population 76 million, is joining the EU”, despite the fact Turkey is no closer to joining it than it was 10 years ago. Johnson has since lied that he did not mention Turkey in the campaign. Vote Leave also claimed that leaving the EU would free up £350m a week that could be spent on the NHS, a claim that has led the UK Statistics Authority to criticise Johnson for “a clear misuse of national statistics”.

The whole political system gets tarred in the long term, while individual perpetrators benefit in the short term

It’s not just the fake figures: Johnson has misled his way through interviews with impunity. He has repeatedly asserted that there will be no customs checks for goods from Northern Ireland making their way to the rest of the UK as a result of his EU exit deal, even as other ministers have been honest that they will indeed be necessary. He has falsely claimed to have reduced murders in London to fewer than a hundred for several years in a row and said that EU immigration represents half of total immigration into the UK (the real figure is around a quarter. And the Conservatives have egregiously circulated an edited clip of an interview of Keir Starmer that misrepresents what actually happened.

There are serious consequences to politicians spreading what can only be termed “fake news”. The problem is that the whole political system gets tarred in the long term, while individual perpetrators benefit in the short term. Misleading the public about how much Brexit will generate for the NHS; making false promises that cannot be kept; putting out sensationalist figures that have no grounding in reality, in the hope they get widely shared on social media: all of these contribute to voters’ lack of trust in politics and a deepening sense of cynicism. It triggers a race to the bottom: if one party is doing it, it paves the way for others to do so too. The Liberal Democrats put out a leaflet two weeks ago that falsely attributed a glowing quote about their prospects to the Guardian.

The more the truth falls by the wayside, the less the incentive for parties to put forward properly costed policies backed by detailed implementation plans. What started with the insidious populism of the Leave campaigns – the idea that Brexit involves no costs – is an election arms race in which parties are churning out uncosted and underdeveloped pledges. Voters are supposed to take it as read that a Conservative government could reverse the regional railway closures of the 1960s for £500m, despite experts saying this is wildly undercosted; or that a Labour government could maintain free access to full-fibre broadband for just £230m a year. These are worthy heirs to David Cameron’s 2010 pledge to cut migration to the “tens of thousands” – now dropped, but only after Conservative politicians spent almost a decade promising to achieve the effectively impossible.

Trust is a precious currency in politics, already in short supply. In recent years, Johnson and his allies have proved too willing to throw it away and it is not just the Conservative party that will feel the consequences.