Wearing face masks has given thieves increased confidence to steal and resulted in more cases of verbal abuse, shopkeepers have told the BBC.
Face coverings became compulsory in stores across England last July as part of the government's plan to reduce the transmission of Covid.
However, some owners said this meant thieves were now harder to detect.
The Association of Convenience Stores (ACS) said 89% of staff experienced verbal abuse during the pandemic.
Neelesh Parekh, who owns a shop in Portsmouth, said he has lost £12,000 at the hands of thieves in the past year and believes mask-wearing has contributed to the problem.
He said: "We can't see their faces - all we can see is their eyes and part of their nose.
"People just take and run - because they've got masks on you can't see what's happening."
Kishor Patel, who runs a convenience store in Basingstoke, added: "We believe that wearing masks in our shops has given the thieves confidence to actually steal things more boldly."
He said he had also been subjected to verbal abuse when challenging customers who were not wearing masks.
A report by the ACS said there had been "horrific incidents of Covid related threats where staff members have been coughed and spat at".
It said the number of shop thefts had remained similar to the previous year, but that this was likely to reflect a "real terms increase" due to there being fewer people in stores because of the restrictions.
The report added, however, that the majority of thefts were still believed to have been committed by "repeat offenders with addiction issues".