Union chiefs have raised "urgent concerns" about the Government's back-to-work plans.
The TUC said proposed guidance expected to be finalised in the next few days places no new requirements on employers to keep people safe at work.
The union organisation said there is no commitment from ministers to increase enforcement to stop companies flouting the law and putting staff in danger.
The TUC said its main concerns include a lack of a binding requirement on employers to ensure safe working conditions.
The guidance repeatedly suggests that "employers should consider" actions such as enabling social distancing or providing handwashing facilities, but also suggests individual employers can decide to ignore the suggestions, said the TUC.
It added there is a lack of any recommendations on personal protective equipment (PPE).
The TUC said unions cannot be expected to support the guidance without knowing what the Government is asking employers to consider in terms of workers' use of PPE.
There is also a lack of safe working requirements for vulnerable groups such as pregnant women, said the TUC, adding that the guidance appears to suggest that pregnant women can be expected to work in unsafe environments, in violation of their existing rights under health and safety law.
TUC general secretary Frances O'Grady said: "Throughout this crisis the TUC has sought to engage constructively with ministers. But we cannot support the Government's back-to-work plans as they stand.
"We want new binding rules for employers to publish their risk assessment and action plan. We want clear guidance to set out the minimum standards that employers must meet in order to protect public safety. And we want ministers to outline a new tough approach to enforcement.
"Unless the Government significantly strengthens its plans, safe working will not be guaranteed. The current proposals fail to provide clear direction to those employers who want to act responsibly. And they are an open goal for rogue employers, who will cut corners and put their workers - and the wider community - at risk.
"We urge ministers to work with the TUC and unions to ensure people can make a safe return to work."
Prospect union general secretary Mike Clancy said: "If the economy is to recover sustainably and safely then a return to work must be managed properly.
"The question is how the advice from Public Health England is grounded in relevant evidence. The Government must provide absolute clarity on how workplaces can operate safely, and it must set out the evidence for its advice.
"We all want to get back to work, but there is no point in easing the lockdown if the guidelines put people at risk, potentially causing a spike in cases and another full-scale lockdown," he added.
John Phillips, acting general secretary of the GMB Union, said: "We desperately need to get the economy going and nobody is keener than GMB to get people back to work - but this guidance was thrown together in a hurry and it shows.
"Giving unions and employers just 12 hours to respond is not good enough and means crucial changes will not be made. We cannot endorse crucial guidance if it is incomplete.
"The guidance has to be clear on how safe working practice is to be enforced.
"As it stands, there is nothing on PPE, nothing on enforcement to ensure workplaces are safe and nothing giving workers the assurances they need to get back to their jobs."