"Robust enforcement" must be taken against employers who are "openly flouting social distancing and lockdown instructions", an assembly member says.
Blaenau Gwent AM Alun Davies wrote to First Minister Mark Drakeford saying prompt action must be taken if the spread of coronavirus is to be stopped.
Many constituents of his have been in touch to say bosses were telling them to turn up for non-essential work.
He cited three examples, including one of a manufacturing company alleged to be ignoring social distancing measures and telling pregnant women previously told to leave work that they should return the following day.
Mr Davies said: "To be candid, I am greatly alarmed at both the number and the nature of the violations that have just been reported to me."
He continued: "These are, however, likely to be an underestimate as many employees are deeply fearful of repercussions if they were to raise concerns."
Workers who have approached him have asked not to be named because they are afraid they will be sacked.
A second case involved a retailer selling non-essential items still opening and reports of significant customer crowding in the store.
The third was a non-essential manufacturer still requiring staff to attend, with members of staff using communal cleaning areas in close proximity to one another.
The letter added: "In addition to these cases, I have also been informed of organisations which are requiring employees who are self-isolating to use their sick leave allocations, despite the fact that they are following explicit government advice."
The Welsh Government has been asked to comment.