Apple has told its global workforce they will not return to its corporate offices until January at the earliest, over concerns about a rise in Covid-19 cases driven by the spread of the Delta variant.

The iPhone maker, which will still keep its network of retail stores open, had previously told staff there would be a phased return to work from October. The delayed office return applies to its international workforce, including those based in the UK.

The company told staff in a memo that it would confirm the reopening plans one month before employees were required to return to the office, according to Bloomberg News.

The memo, sent by the human resources and retail head, Deirdre O’Brien, added that the company did not currently expect to close its offices or retail stores, but she strongly encouraged staff to get vaccinated.

“I know there are feelings of frustration that the pandemic is not yet behind us,” she wrote. “For many colleagues around the world this period has been a time of great tragedy, suffering and heartbreak. Please know that we are all here to support one another and stand with one another during such challenging times.”

Apple, which last month reinstated the mask-wearing policy for in-store staff that it had only decided to scrap in June, had previously planned to ask staff to return to its offices on Mondays, Tuesdays and Thursdays by early September, before delaying that to October. Employees would be able to work remotely on Wednesdays and Fridays.

In the UK, the company employs 7,000 staff and operates 38 retail stores, the most in any country in Europe. Apple’s main UK corporate offices are based in Regent Street, central London, and Stockley Park, near Heathrow.

It is in the process of building new offices on a 17-hectare (42 acres) site at Battersea power station, which will be home to 4,000 staff and was meant to open this year. Apple’s European headquarters are in Cork, Ireland.

The tech giant’s announcement comes as the US faces a new surge in coronavirus infections driven by the Delta variant, with new case numbers averaging more than 130,000 a day after months of decline.

Apple is the latest major corporation to decide conditions are not yet safe enough to expect staff to return to offices.

Earlier this month, Microsoft said it would push back the reopening of its corporate offices from September until at least 4 October.

Amazon staff will remain working remotely until at least 3 January, while Google has extended its voluntary work from home period until 18 October, having previously outlined plans for a September return.

Last month the ride-hailing service Uber pushed its office return back to October, while its rival Lyft does not expect office staff to return until February 2022.

Apple recently said some retail staff could work from home by assisting people online.