Ratcliffe's Ineos Group is expected to pay in the region of £1.25billion for a 25 per set stake in the Old Trafford club, while also acquiring significant control over footballing operations.
There is no a firm timetable for the deal to be finalised, but the PA news agency understands it could come as early as next week, with Ineos optimistic it will be done before the Premier League campaign resumes on November 25 after the international break.
Reports that the deal could come as soon as Monday have, however, been dismissed as that is the date of Sir Bobby Charlton's funeral.
It is almost a year since the Glazer family announced, on November 22, 2022, that they were considering "strategic alternatives" to help the club grow, which included consideration of a sale.
Qatari Sheikh Jassim bin Hamad Al Thani and Ratcliffe emerged from a large field of interested parties, and made offers in the region of £5bn for a complete takeover, but that fell short of the Glazer family's valuation.
Sheikh Jassim then withdrew from the process last month, while Ratcliffe continued to pursue a minority shareholding which could later be increased.
The 71-year-old, who tried to buy Chelsea last year, grew up in the Manchester region and describes himself as a lifelong fan of United.
The deal is expected to lead to a major shake up of United's football operations. Sir Dave Brailsford, the former performance director of British Cycling, is expected to take on significant influence in his role as Ineos's director of sport.
The group already owns French Ligue 1 club Nice and Swiss club Lausanne, as well as the Ineos Grenadiers cycling team and Ineos Britannia - the sailing team led by Sir Ben Ainslie.
They also have a stake in Formula One team Mercedes, whose team principal Toto Wolff last week said he would also consider investing in United once Ratcliffe's deal is completed.
Ongoing speculation and uncertainty over the club's ownership has come at a time when United are struggling on the pitch.
Wednesday's stunning 4-3 defeat to Copenhagen has left Erik ten Hag's side bottom of their Champions League group, while they are eighth in the Premier League, having lost nine of 17 games in all competitions.