Dozens of workers were seen coming into close contact with each other at the warehouse in Stafford in footage that was leaked to the Express & Star amid concerns over welfare.
A worker said teams were coming into contact with "complete strangers from other parts of the region".
They added staff were being kept waiting in line for other shifts to finish.
It is not the first time the company, which is run by its logistics partner Wincanton, has come under fire during the pandemic. Numerous complaints about the treatment of workers last year led to union officials demanding meetings with bosses.
Wincanton has stressed throughout the pandemic that the safety of its staff is its "number one priority".
But the new footage shows staff at the Shackleton Way warehouse lined up against a wall then walking along a corridor with barely any distance between them. It's not clear if any are wearing masks as the faces of workers are obscured.
Screwfix stores currently remain open despite the lockdown, with safety measures in place.
The anonymous worker said: "Wincanton's idea of social distancing is unbelievable. Shifts used to be let out before the next one enters, now, because they want more work out of their employees, the next shift is let in and having to wait for the others to finish. People having to wait in line, next to complete strangers from other parts of the region, not knowing who they have been mixing with.
"This is not acceptable under the present situation, even more so now with the newest strain of the virus being more infectious. People were worried that they might catch it, but now worry about when they will catch it."
Screwfix has been approached for comment.