British billionaire Sir Jim Ratcliffe has agreed to buy a 25% stake in Manchester United for about $1.3bn (£1.03bn).
Ratcliffe's Ineos Group will take control of football operations.
The 71-year-old will also provide $300m (£236m) for future investment into the club's Old Trafford stadium.
The announcement comes 13 months after the club's owners, the Glazer family, stated they were considering selling to "explore strategic alternatives".
The American family bought the club for £790m in 2005.
The only other publicly declared bidder, Qatari banker Sheikh Jassim bin Hamad Al Thani, withdrew his offer to buy 100% of the club in October.
United have struggled on the pitch this season and have not won the Premier League since 2013, while there have been regular protests against the Glazer ownership.
Manchester-born Ratcliffe is chairman of petrochemicals company Ineos and says he is a "lifelong supporter of the club". He tried and failed to buy Chelsea last year..
In a statement, he said: "Whilst the commercial success of the club has ensured there have always been available funds to win trophies at the highest level, this potential has not been fully unlocked in recent times.
"We will bring the global knowledge, expertise and talent from the wider Ineos Sport group to help drive further improvement at the club, while also providing funds intended to enable future investment into Old Trafford.
"We are here for the long term and recognise that a lot of challenges and hard work lie ahead, which we will approach with rigour, professionalism and passion. We are committed to working with everyone at the club - the board, staff, players and fans - to help drive the club forward.
"Our shared ambition is clear: we all want to see Manchester United back where we belong, at the very top of English, European and world football."
The club say the deal is "subject to customary regulatory approvals" but are "hopeful it will be completed as soon as possible".
There is initially going to be a six-to-eight week process for the deal to be ratified, which includes being signed off by the Premier League.
How will the deal work?
Ratcliffe has agreed to buy 25% of the club's class B shares, largely held by the Glazer family, who own 69% of the club, and contain almost all the voting rights, and up to 25% of its class A shares, which are listed on the New York Stock Market. The shares are worth $33 per share.
It means the Glazer family will retain a majority stake in the club.
Taking on sporting control at the club means Ineos will oversee the men's and women's football operations and academies.
They will also get two seats on the Manchester United PLC and Manchester United Football Club boards.
Subject to necessary approvals, Ratcliffe will delegate his seats on the PLC board to John Reece, a shareholder of Ineos, and Rob Nevin, chairman of Ineos Sport.
On the UK club board he will delegate his seats to former British Cycling chief Sir Dave Brailsford and former Juventus and Paris St-Germain executive Jean-Claude Blanc.
United's executive co-chairmen and directors, Avram Glazer and Joel Glazer, said: "Sir Jim and Ineos bring a wealth of commercial experience as well as significant financial commitment into the club.
"Through Ineos Sport, Manchester United will have access to seasoned high-performance professionals, experienced in creating and leading elite teams from both inside and outside the game.
"Manchester United has talented people right across the club and our desire is to always improve at every level to help bring our great fans more success in the future."
Ineos already owns French Ligue 1 club Nice, who are currently second in the table and well placed to qualify for next season's Champions League, and Swiss club Lausanne.
Its sporting portfolio also includes high-profile sailing team Ineos Britannia - led by Sir Ben Ainslie - which is aiming to win the 2024 America's Cup for Great Britain.
Ineos also has a five-year partnership with Formula 1 team Mercedes, owning a third of the team - and took over the British-based Team Sky in cycling in 2019.
'Trawlers Limited' - the company owned by Ratcliffe which is making the acquisition - was inspired by the famous quote by former United striker Eric Cantona.
10 years of struggles
Fans opposed to the Glazers' continued involvement at United have held a series of protests inside and outside Old Trafford over the last 12 months of a process which people close to it confidently predicted would be concluded by the end of last season.
The past decade, since legendary manager Sir Alex Ferguson retired in 2013, has seen inconsistency on the pitch despite a high transfer spend, and several big name managers hired and sacked.
United are again inconsistent this season under Dutch manager Erik Ten Hag, having lost 13 of their 26 games in all competitions, sitting eighth in the Premier League and having finished bottom of their Champions League group and been knocked out of the EFL Cup.
Manchester United Supporters' Trust (Must) said "fans have very mixed feelings today" and that the Glazers' ongoing role was "a cause for serious concern".
The trust added that it wished Ratcliffe's stake was more than 25% and that it is a "puzzle how any organisation can put its very core business in the hands of a minority shareholder, and how that meaningfully works in practice".
"It is now incumbent on the club's owners and management to properly explain how this new structure will work, where the new investment will be directed and how it will benefit the team on the field.
"Today might - just might - be a step forward for Manchester United after some very difficult years. But with the Glazers still in charge, people should understand that United fans will remain sceptical and wait for the proof in the pudding."
Analysis
BBC Sport has been told by people inside and close to the club that a fresh pair of eyes was needed to assess its running.
The launch of the search for strategic alternatives effectively blocked former chief executive Richard Arnold from presiding over change. Now he has gone, Brailsford and Blanc will be doing the assessment. It will be intriguing to see what role interim chief executive Patrick Stewart will play.
On current evidence, they have a lot to go at.
While manager Erik ten Hag is convinced his team are on the right path, results indicate a truth behind something else BBC Sport has been told, that United like to congratulate themselves on doing easy transfer deals and shy away from difficult ones.
In other words, last summer, they signed players like Casemiro - who Real Madrid were happy to let go - for £70m, having failed to get Ten Hag's first choice, Frenkie De Jong. This summer they went for Rasmus Hojlund from Atalanta rather than Harry Kane, who United concluded was too expensive and too difficult to sign. Kane is the leading scorer in Europe's big-five leagues, Hojlund is yet to get a Premier League goal.
Declan Rice has been outstanding for Arsenal this season, a potential game-changer in the title race. United signed Mason Mount, who was out of form before he got injured, and Sofyan Amrabat on loan from Fiorentina, who has not been able to secure a regular place in an unsuccessful team.
It is that kind of perceived wastefulness in the transfer market that has to be addressed.
Manchester United have fallen a significant distance, on and off the pitch, since Sir Alex Ferguson was in his pomp. Their fans can only hope, after false dawns under Louis van Gaal, Jose Mourinho, Ole Gunnar Solskjaer and, now, Ten Hag, this really is the start of the long climb back.
Timeline: How did Manchester United get here?
In January, Ratcliffe's firm Ineos officially stated an interest and said the company had "formally put ourselves into the process".
Ineos and Sheikh Jassim submitted their first bids for United in February, with the Qatari consortium saying their offer "plans to return the club to its former glories".
By March, both parties had entered second, revised bids after a deadline was extended at their request, amid confusion.
Elliott Investment Management and leading private equity group Carlyle, both based in the US, also entered the race but were only willing to purchase a minority stake.
The potential sale reached a third round of bidding in April, with Thomas Zilliacus saying he had also submitted a bid to buy the club.
However, the Finnish entrepreneur declined to enter a another offer, saying he would not participate "in a farce" and instead decided to keep his earlier second bid on the table.
Although the Glazer family did not respond following the deadline on 28 April, there had been increasing confidence in the Ineos camp that their efforts to take over at Old Trafford would be successful after it emerged its bid put a higher overall value on the club than Sheikh Jassim, because he was paying more for a smaller stake.
In May, Sheikh Jassim made an improved fourth bid of nearer to £5bn for 100% of United, in an offer that would clear the almost £1bn debt of the club and included a separate fund for the club and local community.
Sheikh Jassim withdrew from the process to buy United in October after further talks broke down.
Ratcliffe had already amended his initial plan to buy the Glazers' 69% shareholding and reduced it to a 'majority' stake.
However, it was only when he reduced it again, to 25%, that the potential for an agreement increased.
CORRECTION: 24 December 2023 - This story originally reported that the 25% stake was purchased for about £1.25bn, when the correct figure was in fact £1.03bn
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