Employers are warning the chancellor Rachel Reeves that they will cut their staff pension contributions if she imposes national insurance on them.

All employers have to make minimum pension contributions of 3% of staff pay under the auto-enrolment rules, but many pay in more. Both employers and employees’ contributions currently qualify for national insurance relief.

A poll of 600 companies found two in five (42%) of those that pay more than the statutory minimum would reduce their contributions if national insurance was introduced.

The poll, commissioned by the Association of British Insurers (ABI) and the Reward and Employee Benefits Association (REBA) also revealed that introducing NICs on pension contributions could deter three in five (63%) from raising them in future.

Yvonne Braun, policy director at the ABI, said: “We want to see money flowing into pensions to drive growth and we also want employers to be incentivised to provide good pensions for their workers.

“These changes would have a negative impact on both. They would also mean lower retirement standards in the future at a time when we’re already not saving enough for the long term.”

Senior Treasury officials have advised against reducing the 40% tax relief on pension savings given to higher earners because it would disproportionately hit state workers, but other areas of pension saving are not thought to be protected.

Applying national insurance at the full rate of 13.8% could yield a net £16 billion for the chancellor, while even a lower rate of 2% would bring in a couple of billion, said Sir Steve Webb, the former pensions minister, who is now a partner at the pension consultants LCP.

Baroness Altmann, a former Conservative pensions minister, has been a vocal critic of introducing national insurance on employer contributions.

The latest warning comes amid calls for clarity on pension tax.

Helen Morrissey, head of retirement analysis at Hargreaves Lansdown said: “This ongoing speculation about changes to tax-free cash is damaging.

“The chancellor has recognised that businesses need certainty in the taxation environment to make investment decisions. The same is true of our personal finances.

“The government has left people to make impossible decisions about their investments and pensions.

“The sooner changes such as raiding tax-free cash, can be ruled out, the more people can focus on the long term again.”