According to PA Media, the attack has left the English Premier League club unable to fully restore its computer systems and, as of Thursday night, staff did not have access to their email accounts.
The UK’s National Cyber Security Center (NCSC) is assisting United, it said in a statement quoted by PA Media.
CNN has contacted the NCSC but is yet to receive a response.
In a statement sent to CNN, Manchester United said it won’t be commenting on speculation about who may have been responsible for the attack, or what the motives for the attack might be.
“Following the recent cyber attack on the club, our IT team and external experts secured our networks and have conducted forensic investigations,” said the statement.
“This attack was by nature disruptive, but we are not currently aware of any fan data being compromised.
“Critical systems required for matches to take place at Old Trafford remained secure and games have gone ahead as normal.”
United, currently 10th in the Premier League, faces Southampton on Sunday having defeated Istanbul Basaksehir 4-1 in the Champions League earlier this week.