Primark-owner Associated British Foods warned that the loss of sales from store closures could rise to over £1.5 billion if current lockdowns last until the end of March.
But the group said it is confident of strong growth upon reopening.
Unlike most rival retailers, which continue to trade online during lockdowns, Primark - which trades as Penneys here - does not have an online shop.
This means that lockdowns in its main markets in the UK and Ireland and elsewhere in Europe have halted sales as 76% of its retail selling space is shut.
AB Foods said in a statement that if all those stores stayed shut until February 27, then the loss of sales would reach some £1.05 billion, a 55% jump from December 31 when it warned of a £650m loss.
Finance director John Bason said that the hit would rise by an extra £500m if the pandemic kept all those stores closed until the end of March.
He declined to say when he expected lockdowns to end, but said he was confident on future growth.
"I think people will come flooding back to the stores when we reopen," he said.
"We've come through two lockdowns and we're traded really well after both of them," added.
AB Foods, which has suspended group earnings guidance due to the pandemic, said that it now expected Primark's adjusted operating profit to be "somewhat lower than last year", downgrading a December forecast for it to be higher.
AB Foods also has a grocery division, whose brands include Kingsmill bread and Twinings tea, as well as major sugar, agriculture and ingredients businesses.
Those units were performing strongly, it said today, with home baking and high demand for supermarket goods driving growth and it expected their adjusted operating profit to be "well ahead of last year".
Brexit had not caused any material disruption to its supply chains, AB Foods added.