Torrents of water brought down a suspended ceiling and damaged stock in the Victoria Centre store at about 22:40 BST on Tuesday.
Managers had hoped for a weekend reopening but it is now closed "until further notice".
Staff have been helping with the clean-up operation.
Water poured through from a rooftop room, leaving the top floor under three inches of water and stock "significantly" damaged.
A spokeswoman said: "Our teams are working around the clock to get the shop open as quickly as possible and we're sorry for the inconvenience this has caused to our customers."
John Lewis's Nottingham store is to remain closed longer than expected after 80,000 litres of hot water leaked from a ruptured heating pipe.