Frances O’Grady, who leads the group representing UK unions, said it cannot back the advice in its “current form”.
She said there were “huge gaps” over protective kit and testing.
Reduced hot-desking and alternatives to social distancing where it is not possible are among measures being considered by the government.
The document, seen by the BBC, is one of seven draft plans to ease anti-virus restrictions.
It also urges employers to minimise numbers using equipment, stagger shift times and maximise home-working.
The guidance covers the whole of the UK – but the devolved governments have the power to make their own decisions on how businesses can get back to work.
Meanwhile, the number of coronavirus-related deaths in the UK stands at 28,734, an increase of 288, Health Secretary Matt Hancock said at the Downing Street briefing.
The daily increase in deaths is lower than at any point since the end of March, but the figures reported at the weekend tend to be lower and are expected to rise, Mr Hancock said.
A total of 13,258 people are currently being treated in hospital, while 85,186 coronavirus tests took place on Sunday.
However, hospital admissions have fallen, along with the number of critical care beds being used.
A coronavirus contact-tracing app aimed at limiting a second wave of coronavirus will be trialled on the Isle of Wight this week, before being rolled out more widely in the UK, as part of the government’s test, track and trace effort.
Mr Hancock said creating the system was a “huge national undertaking” and would allow the UK to take a “more targeted approach to lockdown while still safely containing the disease”.
Buzzfeed has seen all seven draft documents on getting people back to work.
Ms O’Grady said the Trades Union Congress had seen some of the documents on Sunday.
She said workers’ safety must not be compromised and called for “robust direction and enforcement” so employers can “do the right thing” and action can be taken against those who do not.
She told BBC Radio 4’s The World at One: “The problem is the government is asking us to trust to employer discretion, use words like ‘consider social distancing’, ‘consider having hand sanitiser or soap available’, and frankly that’s just not good enough.”
Asked whether the government’s current advice will compromise worker safety, Ms O’Grady said No 10 has time to “get this right” and it should work with unions to ensure “a proper job” and “not a botched job”.
According to one of the seven draft documents seen by the BBC, firms are told to enact additional hygiene procedures, as well as physical screens, and protective equipment should be considered where maintaining distancing of 2m (6ft) between workers is impossible.
However, the section marked personal protective equipment (PPE) contains only a promise that “more detail” will follow.
‘Roadmap out of lockdown’
During the Downing Street briefing, BBC health editor Hugh Pym asked where those businesses required to have PPE for their staff would source it, and whether they would be in competition with the NHS.
Mr Hancock said the “first call” on PPE must be for NHS and social care staff, as well as those “essential services who need it to keep the people delivering those services safe”.
He reiterated PPE was one of the government’s five tests for adjusting the lockdown.
The BBC has also seen a second document with advice for the hospitality industry, which says bar areas, seated restaurants and cafe areas must be closed, with all food and drink outlets serving takeaway food only.
It adds hotels should consider “room occupancy levels to maintain social distancing, especially in multi-occupancy dormitories”.
It also says “guidance to follow” on the use of PPE and face masks.
Some of the other guidance featured in the document includes:
Allowing access to as few people as possible into kitchens;
Spacing workstations 2m apart as much as possible;
Minimising contact between kitchen and food preparation workers and delivery drivers or riders for example by having zones from which delivery drivers can collect packaged food items;
Using front of house staff to serve customers in walk-in takeaways, with tills 2m away from the kitchen and ideally separated by a wall or partition;
Creating a physical barrier, such as a screen, between front of house workers and customers where possible;
Asking customers to order online, on apps or over the telephone to reduce queues and stagger pick-up times;
Providing handwashing and hand sanitisers and encourage visitors to wash their hands regularly;
Considering room occupancy levels to maintain social distancing, especially in multi-occupancy dormitories.
Taking measures to avoid crowded reception areas, staggering check-in and check-out times.
Boris Johnson is to reveal a “roadmap” out of lockdown on Sunday, but in a video message on Monday he said the the UK must not lift restrictions too soon.
In the video, posted on Twitter, Mr Johnson said: “The worst thing we could do now is ease up too soon and allow a second peak of coronavirus.”
Mr Johnson said the UK would only be able to move on to “the second phase of this conflict” when the government’s five tests had been met, including a sustained and consistent fall in daily deaths and being confident any adjustments would not risk a second peak which could overwhelm the health service.
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.
Ministers are obliged to review those restrictions by Thursday.