“Facts are sacred,” wrote the Manchester Guardian’s editor, CP Scott, in a famous essay marking the newspaper’s centenary in 1921. Propaganda, on the other hand, was “hateful”. No democrat today would publicly disagree with the idea that the news media’s role is to tell the truth. But if accounts of current affairs have always been shaped, to greater and lesser degrees, by values and interests (particularly in wartime, when honesty has often been treated as secondary to security), the pressure now being brought to bear on facts in news and politics, in the UK and elsewhere, is new.

In an election campaign, reliable information matters more than ever. At a time when the British public’s faith in politics is widely recognised to have been stretched to its limits by the Brexit impasse, this is even more the case. This makes the cavalier tactics of the Conservative party over the past week all the more alarming. A few days ago the party released a doctored video in which the shadow Brexit secretary, Keir Starmer, appeared to struggle to reply to a question that in reality (in an interview with Piers Morgan) he had answered straight away. At the weekend, the chancellor, Sajid Javid, and the business minister, Kwasi Kwarteng, both used television interviews to launch attacks on Labour that were based on speculation.

Lacking credible evidence, since Jeremy Corbyn’s party has yet to sign off its manifesto, Mr Javid insisted that Labour plans to spend £1.2tn in government, which would produce “an economic crisis within months”. Mr Kwarteng repeated the claim while refusing to give any details of Conservative spending promises. Labour’s reaction was furious, with the figures dismissed as “fake news” and a “work of fiction”. But while this might appear to be no more than the usual rough and tumble of our intensely adversarial system, the worry is that it is becoming ever harder to hold power to account.

Facebook and the fake news (and fake videos) it facilitates is one culprit. Regulation of online spending, both in the UK and globally, is urgently needed. But to point the finger of blame solely at new media is simplistic. The falsehoods and conspiracy theories for which the internet has proved such fertile spawning ground have hastened the polarisation of our politics and culture. But decline has deeper causes.

Old media must bear its share of responsibility. In the UK, a uniquely aggressive, mainly rightwing tabloid press has long made cynicism about public institutions its stock in trade. Austerity and Brexit have combined to further break down faith in our common store of values and knowledge – a situation that the growing list of disgraced would-be parliamentary candidates can only exacerbate. Meanwhile Boris Johnson appears to revel in evading scrutiny, giving Putin-style Facebook audiences instead of press conferences, disgracefully deferring publication of the intelligence committee’s report into Russian interference in UK politics, and contradicting his own Brexit secretary’s explanation of the future customs arrangements between Northern Ireland and the rest of the UK.

Such claims must not escape challenge, while efforts by journalists, and charities such as Full Fact, to supply voters with accurate information must be redoubled. Attempts to withhold or mislead must be remorselessly exposed. Almost a century ago, CP Scott wrote that the public had “a shrewd intuition of what to accept and what to discount”. In the coming weeks, this newspaper will do what it can to help.