Rishi Sunak has failed to explain when or how he would deliver his plan to abolish national insurance for workers.

Chancellor Jeremy Hunt said in his budget speech yesterday that the current “double taxation” system of employee national insurance (NI) and income tax was “unfair”.

After cutting NI from 10 to 8%, he said his long-term ambition was to “end this unfairness”.

Labour were quick to point out the exorbitant cost of getting rid of employee NI, and suggested it was as “irresponsible” as Liz Truss’s disastrous mini-budget of 2022.

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The Office for Budget Responsibility said income tax raised £251billion last year, while all forms of national insurance brought in £177bn.

Shadow Chancellor Rachel Reeves said Mr Hunt must explain “where the money is going to come from” to abolish NI.

She told BBC Breakfast: “Yesterday, at the end of the Budget, the Chancellor started floating this idea that he was going to get rid of National Insurance altogether.

“Well, that would cost £46bn. And I would like to know where that money is going to come from, because I just wouldn’t make a promise like that without being able to say where the money is going to come from.

“I think it is incumbent on politicians to be honest about the trade offs that have to be made.”

Paul Johnson,director of the Institute for Fiscal Studies, said the proposal to abolish NI was “not worth the paper its written on” unless the Chancellor explained how it will be paid for.

He said: Talk of abolishing national insurance does not look realistic. Let’s be clear, he is not talking about abolishing NI, he is talking about that relatively small part paid by employees.

“But this pledge to cut taxes by more than £40 billion goes in the same bucket as pledges to increase defence spending – not worth the paper its written on unless accompanied by some sense of how it will be afforded.”

In a series of post-budget interviews today, Mr Hunt admitted that abolishing NI would be a “huge job” and it would not happen “any time soon”

Later, Mr Sunak declined to put a cost on abolishing employee NI contributions.

He told broadcasters: “I think what people can see from me, I think they trust me on these things, is that I will always do this responsibly.

“We funded our current tax cuts responsibly, borrowing hasn’t increased, we are still on track to meet our fiscal rules that have our debt falling.”

The Prime Minister said he wanted to “make progress” on axing NI in the next Parliament.

Speaking at an event at a pub in South Yorkshire, Mr Sunak said: “The reason we have chosen to cut national insurance in particular is because it’s a tax on work.

“We have this unfairness at the moment where if you’re working, you pay tax twice, once in income tax and then again in national insurance.

“That’s unnecessarily complicated because all of that money ultimately goes into the same pot, funds the same public services. But ultimately, as I said, it’s unfair because you are paying tax not once but twice.

“But my ultimate ambition is to remove that unfairness entirely, and if we stick to our plan, not just will we deliver the £900 of tax cuts this year, we can really make progress towards that long-term ambition over time in the next Parliament.”

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Mr Johnson said the next parliament could be the hardest for 80 years for reducing UK debt.

He said: “The combination of high debt interest payments and low forecast nominal growth means that the next parliament could well prove to be the most difficult of any in 80 years for a chancellor wanting to bring debt down.

“Even stabilising debt as a fraction of national income is likely to mean some eye-wateringly tough choices, and we are talking tens of billions of pounds worth of tough choices, on tax and spending.”

He also said both the Tories and Labour were engaged in a “conspiracy of silence” about the grim state of the public finances after the election.

He said: “This was not a Budget which addressed the real challenges we are facing because it was not transparent about what those challenges are.

“Government and opposition are joining in a conspiracy of silence in not acknowledging the scale of the choices and trade-offs that will face us after the election.

“They, and we, could be in for a rude awakening when those choices become unavoidable.”

The International Monetary Fund also said stabilising the UK’s debt was likely to require additional tax rises.

 Significant spending to protect service delivery, growth-enhancing investment and the appropriate commitment to stabilise debt are likely to require additional revenue-raising measures in the medium term,” the IMF said.