How did the Conservatives get away with inventing and publicising a ridiculous figure for Labour spending? Their claim that Labour’s plans would cost £1.2tn included many things the party ruled out doing, such as abolishing public schools; it doubled the cost of policies such as bringing the National Grid into public ownership; it counted 10-year commitments (such as making housing more energy-efficient) as happening over five years; and so on.

Exercises of this kind have a long history, but they have become more potent since the global financial crisis. This led to a rising government deficit that the Tories could pretend was a consequence of Labour profligacy, whereas in reality it was the result of the largest recession since the second world war. With the help of their friends in the City – including Mervyn King, the Bank of England governor at the time – they managed to convince enough voters and the Liberal Democrat leadership that the rising deficit was a crisis that required immediate austerity.

It was nonsense, with no data to actually indicate a crisis. Deficits always rise in recessions, and it is basic economic wisdom that the last thing you do in that situation is cut spending, because you will choke off the recovery. That is exactly what happened between 2010 and 2012 as a result of austerity.

Despite this colossal mistake, which probably cost the average household something like £10,000 in lost resources, most of the media bought the idea that Labour’s overspending was to blame, and that austerity was now necessary. That meant, in turn, that many voters began to believe it.

From then on, the Conservatives had a well of mistrust against Labour spending that they could tap into during any general election. This is what lies behind their latest outrageous estimate. It was always meant to be rubbish. But the Tories knew their supporters in the press would still put it on the front page – and the more the opposition started to dispute the numbers, the more the meme of Labour overspending would remain in the headlines.

The classic example of this trick was the leave campaign’s £350m-a-week claim during the 2016 referendum. And this week, we have had bogus claims about immigration under Labour. If they distract from NHS waiting times, the lies will have done their job.

This trick always works in terms of feeding lines to the rightwing press. Whether it works for the broadcast media depends on how the broadcasters respond. They dealt with the Labour spending claims pretty well, because the Conservatives had not costed their own spending. But that didn’t stop the Conservatives getting a totally spurious number into the public domain and making the issue a topic of debate. The best way for the broadcast media to deal with these kinds of made-up numbers is to ignore them.

When it comes to the Conservatives’ own policies, though, the media should be challenging the figures. There has been a failure to engage properly with the details of NHS funding. Basic questions about claims involving doctor or nurse numbers, such as what numbers per person in the population might look like, are rarely asked. Few in the media seem to understand that the share of NHS spending in GDP needs to rise each year to cope with an ageing population and other factors, so “protecting” the NHS in spending rounds actually meant a gradually worsening service.

Equally there still seems to be a general belief in the media that the Tories can manage the economy better than Labour. I understand that journalists who are not experts in macroeconomics find it hard to challenge the constant Conservative narrative of a strong economy, so here are some very basic figures that should help.

The average annual growth in GDP per head, which is a better measure of how much prosperity is growing than GDP growth, was just above 1.7% during the Labour government years (1997 to 2009). And if we take the average to 2007 rather than 2009, excluding the years of the global financial crisis, we get over 2.5%. The same figure for 2010 to 2018 is under 1.2%; the third quarter of this year, compared to a year earlier, was only 0.4%. The economy under a Conservative chancellor since 2010 has been weak, not strong.

The truth about politicians lying is that it only works for as long as the media let them get away with it.

• Simon Wren-Lewis is emeritus professor of economics and fellow of Merton College, Oxford. He blogs at Mainly Macro