The general secretary of the Unite trade union has said workers "should refuse" to return to work if there is no "safe environment" for them.
Len McCluskey was speaking to the BBC after Boris Johnson unveiled a "conditional plan" to reopen society.
The prime minister said those who could not work from home should be "actively encouraged to go to work" in England.
Meanwhile, business groups have called for clarity on what will need to change in the workplace.
Mr McCluskey said it was every worker's "statutory right" to have such an environment and any worker "unsure" of having that available "should not be pressured in going back to work".
He also said that he did not believe there should be any "need for that", as long as "employers and government embrace expertise".
He added that the economy had to be restarted: "Otherwise we'll be faced with mass unemployment, which will impact on everybody."
Business groups including the British Chambers of Commerce (BCC) have urged the government to provide clear guidance on the relaxation of coronavirus lockdown restrictions.
He added: "It is imperative that companies have detailed advice on what will need to change in the workplace, including clarity on the use of personal protective equipment (PPE)," said BCC director Adam Marshall.
Federation of Small Businesses national chairman Mike Cherry said: "Small businesses will need time to adapt after the workplace guidance is published, and for smaller businesses it must be proportionate and focused on the overall outcome of maintaining safe working environments, achieved as straightforwardly as possible."
Clear guidance 'vital'
In a televised address, Mr Johnson said he wanted those in the construction and manufacturing industries to return to work this week.
Caution was, however, urged by other trade groups, such as the Institute of Directors (IoD).
Its director general, Jonathan Geldart, said it was vital the guidance was clear so that companies could plan how to return safely.
"As people with ultimate legal responsibility, directors need to have confidence that it's safe, and that if they act responsibly they won't be at undue risk. Businesses should consult with their people to put in place robust policies, which in many cases might not be an overnight process."
Carolyn Fairbairn, the director general of the Confederation of British Industry (CBI) said businesses were "keen to open and get the economy back on its feet".
"But they also know putting health first is the only sustainable route to economic recovery. The message of continued vigilance is right," she said.
She added: "While stopping work was necessarily fast and immediate, restarting will be slower and more complex. It must go hand in hand with plans for schools, transport, testing and access to PPE. Firms will want to see a roadmap, with dates they can plan for."
Difficult path ahead
Very little has changed in terms of the regulations and prohibitions first announced in March. The lockdown remains. Closed shops, for example, won't reopen.
What we did get was a "change of emphasis" - that people should assume they should go back to work, rather than presume they should not.
Some in government and in industry fear that the "stay at home" message has now deeply embedded itself in the minds of millions of workers.
The prime minister's replacement of that message with "stay alert" in England is designed to get businesses to use the existing discretion in the lockdown regulations.
The practicalities of that are not easy, however, with business groups and unions not agreeing on what constitutes a "Covid-safe" workplace.
The government acknowledges that there won't be enough public transport options for people to return to work. Many workers will also face problems with childcare.
On top of that, to the extent that some industries will reopen - such as construction and perhaps some forms of hospitality by July, any increase in the rate of infection could see the brakes applied quickly.
The path ahead will be a delicate, difficult and constant balancing act between health and the economy. For now, the economy is definitively second priority.
During his address, Mr Johnson added that workplaces would receive guidance on how to become "Covid secure".
Frances O'Grady, general secretary of the Trades Union Congress, called for those guidelines to be published.
Lots of working people will feel confused and anxious after listening to Boris Johnson.— Frances O'Grady (@FrancesOGrady) May 10, 2020
Govt still hasn’t published guidance on how workers will be kept safe. So how can the PM – with 12-hours' notice – tell people to go back to sites and factories?
It’s a recipe for chaos.
BBC News previously reported that reduced hot-desking and alternatives to social distancing where it is not possible were among measures being considered to let workplaces reopen.
One of seven draft plans to ease anti-coronavirus restrictions, seen by the BBC, also urged employers to minimise numbers using equipment, stagger shift times and maximise home-working.
Many companies have been shut since widespread limits on everyday life were imposed on 23 March, in a bid to limit the effects of the virus's spread on the NHS.
As a result, the government is now paying the wages for nearly a quarter of UK jobs under a programme aimed at helping people put on leave due to the virus pandemic.
Under the job retention scheme, it funds 80% of workers' wages, up to £2,500 a month.
On Sunday, business groups also urged caution when it came to any future withdrawal of the support.
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