British billionaire Jim Ratcliffe's acquisition of a 25 per cent stake in Premier League club Manchester United has been completed, putting an end to a 15-month saga.

The $1.25 billion deal, in which the INEOS chairman will also invest $300 million into the club's infrastructure and take charge of their football operations, was struck in December and its final approval was mainly a formality.

"To become co-owner of Manchester United is a great honour and comes with great responsibility," Ratcliffe said in a statement on Tuesday.

"This marks the completion of the transaction, but just the beginning of our journey to take Manchester United back to the top of English, European and world football, with world-class facilities for our fans. Work to achieve those objectives will accelerate from today."

The ownership deal had to clear regulatory hurdles, including Premier League and Football Association approval which were both given last week.

Ratcliffe's purchase ended more than a year of uncertainty after majority owners, the Glazer family, said in November 2022 that they were looking at strategic options for investment into the club they took control of in 2005.

The American family will retain a majority stake in the club but Ratcliffe's Ineos Group will take control of football operations.

"I would like to welcome Sir Jim as co-owner and look forward to working closely with him and INEOS Sport to deliver a bright future for Manchester United," executive co-chairman Joel Glazer said in a statement.

Ratcliffe has agreed to purchase 25 per cent of the team's class B shares, largely held by the Glazer family, which holds most of the voting rights, and up to 25 per cent of its class A shares, which are listed on the New York Stock Exchange.

Ratcliffe will inject a further $300m for investment in infrastructure at Old Trafford that will acquire him a further 3% share later this year.

The deal makes him the largest individual United shareholder. His company Trawlers Limited was used for the purchase.

The 71-year-old Ratcliffe, who grew up a United fan, told reporters recently that ownership in the club was the most exciting endeavour of his life.

The Glazers have been heavily criticised by fans who blame them for a steady decline in the fortunes of the 20-time English league champions who are currently sixth in the standings.

While many United supporters had hoped the Glazers would sell the club in full, Ratcliffe's investment has been welcomed. Since the Christmas Eve deal, Ratcliffe has brought in right-hand man Dave Brailsford and Jean-Claude Blanc, who join him on the board.