Senior business leaders and trade bodies have backed Keir Starmer’s comments that Britain should not part from the European Union on standards ranging from the environment to employment.

The Labour leader has come under fire from the Conservatives, who accused him of wanting to “unpick” Brexit after saying that “most of the conflict” since 2016 had arisen because the UK “wants to diverge and do different things to the rest of our EU partners”.

In a letter to the Guardian, dozens of business leaders and trade bodies joined with political figures hostile to Brexit in endorsing what they described as a “policy of alignment with EU standards and regulations, unless it is explicitly not in the UK’s interests to do so”.

Signatories to the letter included Peter Norris, the chair of Virgin, and Paul Drechsler, from the International Chamber of Commerce (ICC), who said that such a policy would “enable businesses and investors to have confidence in the UK’s regulatory foundations, while still allowing the UK to maintain its own regulatory autonomy”.

The letter adds: “By taking this approach, any future UK government will be able to reassure businesses and investors, both domestically and internationally, that the UK is committed to maintaining high standards and protections, strong relationships with its trading partners, while also protecting the interests of UK businesses.”

The position of “beneficial alignment” differs from “dynamic alignment” – the idea that the UK would follow evolving EU rules in an area indefinitely, which Labour is at pains to stress that it does not support.

Other signatories to the letter included Richard Griffiths, chief executive of the British Poultry Council, and Steve Brambley, chief executive of Gambica (the UK Trade Association for Instrumentation, Control, Automation and Laboratory Technology). The letter was organised by Best for Britain, which was originally launched in 2017 to oppose Brexit.

Norris said: “Leaving the single market has already increased costs for businesses and consumers and further divergence will only hike prices further during a cost of living crisis.”

The intervention comes before the Labour party conference in October, where Starmer is also likely to come under greater pressure from members of his own party who are eager for him to take an even more forthright position on deepening ties with the EU.

External groups such as Best for Britain have also been keen to add to the political argument that polling shows alignment in UK-EU relations is the most popular option in all but six constituencies in England, Wales and Scotland.

Starmer’s comments, made on a visit to Canada at an event bringing together progressive leaders, opened him up to attack from the Tories and prompted surprise from some in his own party who had come to regard Brexit as dangerous territory.

The chancellor, Jeremy Hunt, told LBC radio: “I think those kinds of comments about not wanting to diverge will worry a lot of people that what he really wants to do is to unpick Brexit.”