Ocado has become the first major British grocer to warn of shortages as it grapples with post-Brexit red tape and staff absences caused by the coronavirus. The online shop, which received a huge sales boost since the start of the pandemic as shoppers shunned supermarkets, warned customers that because of changes in the supply chain, they may not receive all goods they order. The store has also been affected by workers taking time off work as they self-isolate after testing positive for the virus. “Staff absences across the supply chain may lead to an increase in product substitutions for a small number of customers as some suppliers consolidate their offering to maintain output,” an Ocado spokeswoman said. Supermarket groups are also concerned that complex customs procedures in place since Britain left the European Union could affect supplies. Last week, Sainsbury’s and Morrisons, Britain’s second and fourth-largest supermarket chains, said goods were flowing from continental Europe so far. But they said traffic volume would not return to normal levels until the third week of January, when new customs procedures will have been properly tested. Marks & Spencer’s chief executive, Steve Rowe, said last week that the shop’s Percy Pigs, a popular sweet, could be subject to import taxes under EU “rules of origin” conditions for countries outside the bloc. There were also reports of hauliers being turned back from the borders with France and Ireland because their customs paperwork had not been filled out correctly.