A Conservative Treasury minister has been mocked by a live audience for refusing to set out the impact of the party’s spending plans – despite attacking Labour for its proposals.

Rishi Sunak was openly laughed at after failing to say – four times – what the “cost of Johnson” would be, after releasing a much-ridiculed dossier on the “cost of Corbyn”.

Asked whether higher Tory spending would also push up voters’ taxes, he said repeatedly: “We don’t think we should borrow for day-to-day spending.”

Amid loud laughter, the BBC Radio 4 presenter Justin Webb, carrying out the interview in front of students at the University of Bristol, 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,” Webb asked.

“I’m asking you just to say openly what you believe the costs of your plans will be.”

The clash came after the Conservatives were accused of peddling “fake news” after claiming a “huge black hole” in Jeremy Corbyn's plans would cost each taxpayer £2,400 in higher bills.

That followed a claim at the weekend that Labour is plotting an “eye-watering” £1.2 trillion of spending over five years, something Mr Corbyn's party has branded an “incompetent mish-mash of debunked estimates and bad maths”.

Analysts have pointed out that this tots up proposals dating back many years – before the release of Labour’s manifesto for this election – and ignores planned tax hikes on high-earners and corporations.

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Both 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.

Last week, the Treasury blocked what was expected to be a damaging new analysis of the state of the economy.

During the interview, Mr Sunak also refused to answer – again, four times – whether a Conservative government would spend “more or less” than Labour on the adult social-care crisis.

Labour has pledged free personal care for all, at an estimated cost of £6bn, while Matt Hancock, the health secretary, hinted Tory plans would be kept under wraps.

“I can’t tell you without looking at the plans that Labour has,” Mr Sunak said, when asked if Conservative spending would be higher.

After claiming Labour’s plans would cost each taxpayer an extra £2,400 each year, he said: “We will not borrow for day-to-day spending.

“The Office for Budget Responsibility forecast shows that there are £30bn surplus on the current balance over the next few years.