Leaving the European Union would create a "melting pot of uncertainty" in UK workplaces, according to the head of the conciliation service.
Sir Brendan Barber, chairman of Acas, said much of the UK's employment legislation and practice comes from the framework of agreed European law accumulated over decades.
The impact on workplaces of any decision to leave the EU in the forthcoming referendum will be "profound", he said in a new year's message.
Mr Barber also predicted a number of "momentous" changes for the world of work in 2016, including the £7.20 an hour national living wage and preparations for an apprenticeship levy to be introduced in 2017.
He said: "From Acas' perspective, more attention needs to focus on the quality of workplace relations as a central driver of business performance and innovation.
"To support that, Acas will be launching, early in the new year, an accessible productivity toolkit to help businesses assess their strategies for boosting workplace performance.
"Britain's economy, while showing signs of recovery, will continue to be buffeted by changing pressures from the global economy.
"Nearer to home, continuing pay restraint across the public sector may also see challenging disputes in hard-pressed public services."
He added that 2016 could be the year when the UK's relationship with Europe was determined.