The trade union said that far too often 'shadow shifts' are crossing over into labour exploitation, with prospective applicants often working excessive hours without pay.
Undertaking, or being invited to do a voluntary trial shift is currently legal, with smaller businesses often relying on these as part of their recruitment processes.
Today (16 March) a private members' bill, which seeks to make unpaid trial shifts illegal, will have its second parliamentary hearing.
The Federation of Small Businesses (FSB) has countered, however, that trial shifts are a valuable way of testing a candidate's suitability for a role. The body has stated though that this should not cross the line into exploitation.
"Small businesses can sometimes be reticent about hiring or even looking to expand headcount when the work is there because they are worried about making the wrong decision," said Colin Borland, senior head of external affairs at FSB.
"The more that we can do to make sure that they hire the right people the better. You just have to be very careful that it doesn't cross into what's exploitative."
Tony Hyams-Parish, an employment expert and partner at city law firm DMH Stallard, said that employers should be clear about the duties involved in an unpaid trial shift during the recruitment process.
“The difficulty candidates have when asked to do ‘free shifts’ is that it will be put to them that it is simply part of the recruitment process," he said. "If that is genuinely the case they should be ‘supernumerary’ and shadowing other staff.
"In such circumstances there is no entitlement to be paid anything. Presumably they will also be observed during the shift."
He added that the difficulty comes when candidates find themselves undertaking an actual job. This could even take the form of "filling in for absent employees [where] there is no sense that they are shadowing or being observed at all," he said.
"The national minimum wage legislation is unclear here but there is some possibility candidates could fall into the category of workers," Hay Parish advised. "Certainly candidates that are asked to do several shifts over a period of time are more likely to be classed as workers and have an entitlement to be paid, but it will depend on the circumstances of each case.”