Mr Javid was continuing to use the hotly-disputed figure in emails to voters on Tuesday evening, despite hearing a live radio audience laugh openly at Treasury minister Rishi Sunak as he failed to produce an equivalent figure for the cost of Conservative plans.
Labour and the Tories are at loggerheads over their economic offers to voters, after the Conservatives produced their estimate of the five-year cost of Jeremy Corbyn’s plans before the party’s manifesto had even been agreed.
Mr McDonnell blasted the headline figure as “ludicrous” and dismissed as false follow-up claims that it would add £2,400 to the average worker’s tax bill. The tax figure was obtained by dividing the Tory estimate equally between all of the country’s taxpayers, while Labour insists that hikes in income tax will be limited to the top 5 per cent of earners.
Independent analysis of the Tory claims by the Fact Check charity found that many of the figures behind their estimate were “uncertain or based on flawed assumptions”, including pledges from a previous manifesto, double counting and the inclusion of policies which Labour denies it is adopting or would phase in over many years.
The row echoes disputes during the 2016 EU referendum campaign over claims by Boris Johnson and Vote Leave that the UK handed over £350m a week to Brussels. Although the figure was ruled “misleading” by the official statistician, the pro-Brexit campaign continued to use it.
Vote Leave chief Dominic Cummings – now a senior adviser at No 10 – admitted later that roughly half of the £350m did not go to the EU, but said the campaign continued to use it “to provoke people into argument” about the cost of Europe, adding: “This worked much better than I thought it would.”
The chair of the UK Statistics Authority Sir David Norgrove wrote last week to the leaders of all the main parties reminding them of the misuse of statistics in the referendum campaign and urging them to ensure any figures they produce are reliable.
“Statistics can be a powerful support for an argument but misuse damages their integrity, causes confusion and undermines trust,” warned Sir David.
“It can also lead debate to focus too much on the statistics themselves, distracting from the issues at hand. This is particularly important during the intense public scrutiny of an election campaign, where misinformation can spread quickly.
“I would ask that statistical sources should be clear and accessible to all; any caveats or limitations in the statistics should be respected; and campaigns should not pick out single numbers that differ from the picture painted by the statistics as a whole.”
The UKSA said it had not yet received any complaints about the Tory dossier.
Mr Sunak was subjected to loud laughter from students at the University of Bristol as he failed four times to say what would be the “cost of Johnson” after he claimed on live radio that the disputed figure represented the “cost of Corbyn”.
Asked on BBC Radio 4’s Today whether higher Conservative spending would also push up voters’ taxes, he said repeatedly: “We don’t think we should borrow for day-to-day spending.”
Presenter Justin Webb protested: “That wasn’t my question. You have just put a figure on what you say the cost of Corbyn will be to individual families. I’m asking you what is the cost of Johnson. I’m asking you just to say openly what you believe the costs of your plans will be?”
Mr Javid came under fire after Channel 4 confirmed this weekend’s planned debate had been put on ice after they failed to reach an agreement with all the parties.
In a video challenge to Mr Javid to take part in the TV showdown, Mr McDonnell said: “He is terrified of debating the truth about Labour’s policies.
“He has been putting forward silly fictitious fake news figures that we will rip apart.
“Let’s make it absolutely clear. We will be bringing forward our manifesto. It will be fully costed, all the funding sources identified. And yes, only the top 5 per cent of earners will pay a bit more income tax. Ninety-five per cent of our people will not face any increase in income tax, VAT or national insurance.”
The Scottish National Party’s Westminster leader Ian Blackford urged the broadcaster to “empty chair” the Tories if they refuse to attend the debate.
“It’s frankly embarrassing that the Tory chancellor is doing a chicken run and refusing to defend his party’s record,” he said.
“The debate must go ahead – it would be a huge disservice to voters if broadcasters allowed the Tories to shut down debate and rig media coverage of the election in their favour.”
Sir Ed Davey, the Lib Dem Treasury spokesman, had agreed to take part before the debate was put on hold, a party source said.
The source said: “First they won’t put their manifesto to the OBR (Office of Budget Responsibility), and now they won’t put their chancellor to the public. It’s clear the Tories are running scared.”
A Channel 4 spokesperson said: “Plans for a chancellors/economy debate are currently on hold as it was not possible to reach an agreement with all parties.
“If the situation does change, we would certainly explore options on how best to proceed. We will be announcing our election plans in due course.”
There was no response from the Conservative Party to suggestions that Mr Javid was blocking the broadcast.
Both major parties are planning big spending rises to end austerity, although Labour’s – at an estimated £55bn a year higher, against the Tories’ £20bn – are far more ambitious.
However, Labour has promised its manifesto will be fully costed, while the Conservatives have rejected calls for all spending plans to be vetted by the Treasury’s independent watchdog.
The Treasury also blocked, last week, what was expected to be a damaging new analysis of the state of the economy.