Britain’s biggest recruitment and staffing companies have written to the government to protest against plans to replace striking workers with agency staff, warning that this would further inflame strikes.

In a letter to Kwasi Kwarteng, the bosses of 13 companies including Hays, Adecco, Randstad and Manpower called on the business secretary to reconsider plans to repeal a decades-long ban on using agency workers to cover for picketing staff.

“We can only see these proposals inflaming strikes – not ending them,” the 13 groups warned in their letter, which was sent by Sarah Thewlis, chair of the Recruitment & Employment Confederation (REC).

The company executives said they felt “compelled” to write to the business secretary as the leaders of the largest staffing businesses in the UK to express their “concern” at how the government was moving ahead with the proposals, which they described as “unhelpful”.

Faced with strikes by thousands of rail workers and staff in other industries over pay offers well below the 9.1% inflation rate at a time of soaring living costs, the government this week put forward draft legislation.

It would remove the restriction under regulation 7 of the 2003 conduct regulations “preventing employment businesses from introducing or supplying agency workers to hirers to replace individuals taking part in official strike or official industrial action”.

The prohibition to apply agency staff during a strike was first introduced in 1976 under the Labour government. The rules are separately enacted in Wales and the UK government intends to repeal them against the wishes of the Welsh Labour government.

The staffing agencies said in the letter: “We strongly believe it has the potential to cost our businesses – as we will be held responsible for sending strike breakers across a picket line and putting our workers in harm’s way. It will not matter if our individual businesses choose not to supply – the industry will be called into disrepute.”

The companies noted that the industry contributed nearly £40bn to the UK economy every year, and was keen to support future growth.

The REC said it had not yet received a response from Kwarteng.

The Department for Business, Energy and Industrial Strategy said: “The business secretary makes no apology for taking action so that essential services, such as train lines, are run as effectively as possible, ensuring the British public don’t have to pay the price for disproportionate strike action.

“Allowing businesses to supply skilled agency workers to plug staffing gaps does not mandate employment businesses to do this. Rather, this legislation gives employers more freedom to find trained staff in the face of strike action if they choose to.”

The government argues that the ability of a business to use agency workers does not stop an individual’s ability to strike; and that agency workers are under no obligation to accept a role replacing staff during strikes.