The row came ahead of Sir Keir Starmer’s showcase investment summit at the Guildhall on Monday, where he will make his “pitch for Britain” by pledging to rip up red tape that “needlessly holds back investment”.

Hundreds of international business leaders and investors are expected to attend the conference before an exclusive reception at St Paul’s Cathedral, attended by the King.

Last week, Sir Keir refused to rule out a hike in NI employer contributions when challenged by Rishi Sunak in the Commons.

Levying NI on employer pension contributions at a flat 13.8 per cent rate would raise up to £18 billion a year by the end of the decade, according to recent research by the Resolution Foundation think tank.

Experts calculate that taxing employer pension contributions could cost high-earners £1,800 a year. Employers pay NI of up to 13.8 per cent on employee earnings, but salary paid into a pension is tax-free.

It comes after Rachel Reeves, the Chancellor, softened a planned crackdown on non-doms amid warnings that the move would cause wealthy individuals to leave the country.

Meanwhile, other measures she expected to fill a black hole of £22 billion, such as raising capital gains tax to as high as 39 per cent and closing a private equity tax loophole, are also expected to raise less than predicted – fuelling fears that she will turn to taxes on working people.

Business leaders warned that an NI hike on employers would hinder growth – one of Sir Keir’s top priorities.

Charlie Nunn, the Lloyds Bank chief executive, said: “Anything that helps people continue to invest and take appropriate risk, we think, is really important. Anything that does the opposite would be a handbrake.

“Pensions, and contributions to pensions, are critical. We see about 40 per cent of people in the UK have a pension which won’t give them even a basic living allowance when they retire. So we need to increase enrolment and investments in pensions.”

Kate Nicholls, the chief executive of UK Hospitality, said: “This is a tax on jobs. An increase in NICs makes it harder to employ people and to take a risk on recruitment and expansion, because the costs of it will be so much higher.”

Lord Spencer, the billionaire investor and a Tory donor, accused Labour of breaking its promise not to tax working people.

“It’s a breach of the principle of their commitment not to change income tax rates,” he said. “They said they weren’t going to change National Insurance and they’re now redefining it and saying they weren’t referring to corporate National Insurance.”

Lord Clarke, the former chancellor, said both Labour and the Tories had been “irresponsible” at the election to promise “not to put up any of the normal sources of tax revenue”.

“By ruling out raising any of the basic taxes that are normal and fair ways of raising tax, they are now having to look at some of these unattractive alternatives,” he told The Telegraph. “Raising National Insurance will have an adverse effect on employers creating jobs and affect the financial position of companies.”

Mr Jenrick, a former exchequer secretary to the Treasury, said: “This is the strongest indication yet that Labour will break another manifesto pledge. All tax is ultimately paid by working people.

“The prospect of this tax on jobs is already scaring away investors, and leaves Starmer’s investment summit in disarray. Labour’s high tax and spend agenda will drive our economy into the ground.”