Cybercriminals targeted Manchester United's IT systems in a "sophisticated" hacking operation, the club said.
"The club has taken swift action to contain the attack and is currently working with expert advisers to investigate the incident and minimise the ongoing IT disruption," it said in a statement late Friday.
All "critical systems" required for games to take place at Old Trafford were secure, the statement said, adding Saturday's game against West Bromwich Albion would go ahead as planned.
"We are not currently aware of any breach of personal data associated with our fans and customers," it added.
"These types of attacks are becoming more and more common and are something you have to rehearse for," a spokesman for the club told the PA news agency.
United are languishing in 14th in the Premier League table and a fine start to their Champions League campaign took a knock with a 2-1 defeat to Istanbul Basaksehir.