It was once Labour, not the Conservatives, that was the UK’s principal eurosceptic party. Like David Cameron, Harold Wilson staged a referendum in a bid to ameliorate his side’s divisions. But though the UK voted to remain in the European Economic Community by 67-33 in 1975, Labour advocated immediate withdrawal just six years later under Michael Foot. For many socialists, among them Jeremy Corbyn, the community was regarded as little more than a capitalist club.

The sea change came in 1988 when European Commission president Jacques Delors addressed the TUC and vowed to put workers’ rights at the heart of his project. His speech was received with a standing ovation and a spontaneous chorus of “Frère Jacques” (which brought tears to the Frenchman’s eyes). Labour had learned to stop worrying and love Europe.

Delors’ intervention prompted Margaret Thatcher’s epochal Bruges speech in which she declared: “We have not successfully rolled back the frontiers of the state in Britain only to see them reimposed at a European level”. For her vanquished domestic opponents, this was precisely the attraction.

Ever since, the issue of workers’ rights has been at the centre of the European debate. During the referendum, pro-Brexit cabinet minister Priti Patel boasted: “If we could just halve the burdens of the EU social and employment legislation we could deliver a £4.3bn boost to our economy and 60,000 new jobs.” From the Remain side, TUC general secretary Frances O’Grady warned that workers’ rights were “on the line”.

Yet the degree to which Brexit will affect employees is unclear. Some of the most significant rights, such as equal pay, sex and race anti-discrimination laws and maternity leave, predate the UK’s European membership. Yet others, such as free movement, paid annual leave, health and safety protection and equal rights for part-time, fixed-term and agency workers, were achieved afterwards. Potential future measures include the posted workers’ directive (ensuring employees moved from one member state maintain the same rights) and greater protection for those on zero-hours contracts.

Were the UK to retain single market membership (as non-EU member Norway does), existing rights would be automatically retained. But the statement by the government that it intends to avoid an “off the shelf” model likely negates this option.

In practice, however, ministers may make few if any changes to workers’ rights. The UK already has one of the least regulated labour markets in the western world, ranking 31st out of 34 OECD countries. David Davis, the Brexit Secretary, recently met with O’Grady and assured her that workers’ rights would not be diluted. Theresa May has emphasised her commitment to “social justice” and has vowed to strengthen workers through representation on company boards.

But she has yet to explicitly guarantee that EU social protections will be maintained. In a letter to May on 30 August, the former shadow business secretary Chuka Umunna wrote: “You have said repeatedly that ‘Brexit means Brexit’. But you must now begin to set out what this means. You owe it to the working people of Britain to make clear that the pledges made by your cabinet colleagues to retain EU legislation on workers’ rights will be delivered.

“Anything else will be a betrayal of British workers, whether they voted to leave the EU or to remain a member. Do the right thing by the working people of Britain, and commit unequivocally to protecting the rights on which dignity at work depends.”

The weakness of Labour, the party founded to represent workers’ interests, may embolden Conservative free marketeers. But more centrist Tories will argue that they should instead claim their opponents’ territory. For UK workers, the greatest effect of Brexit may not be legislative action but the deleterious economic consequences that many predict will result.