Jim Ratcliffe has asked Manchester United's supporters for "time and patience" to bring the glory days back to Old Trafford after the British billionaire struck a deal to take a minority stake in the Premier League club.

Ratcliffe will take a 25% stake in United and pledged to invest $300 million in the club to try to revive their fortunes. The deal is subject to a regulatory sign-off.

United have not won the league since 2013 and although there is hope Ratcliffe can turn things around when he is given the responsibility of football operations, the 71-year-old said there was no quick fix.

"I believe we can bring sporting success on the pitch to complement the undoubted commercial success that the club has enjoyed," Ratcliffe said in an open letter to the Manchester United Supporters Trust.

"It will require time and patience alongside rigour and the highest level of professional management.

"You are ambitious for Manchester United and so are we. There are no guarantees in sport, and change can inevitably take time but we are in it for the long term."

United have fallen to eighth in the standings after eight defeats in 18 games this season. They play Aston Villa later on Tuesday (Wednesday 2:00 am, Bangladesh time).