Potential bidders for Manchester United believe the club's owners, the Glazer family, are aiming to secure a partner investor at Old Trafford rather than a full sale of their controlling stake in Premier League team, sources have told ESPN.

Raine Group, the New York-based investment bank, are overseeing the process to either sell the club or attract substantial new investment and a soft deadline of Feb. 17 has been set for potential investors to register their interest.

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Although there has been reported interest from Qatar and Saudi Arabia in buying or investing in United, Jim Ratcliffe, Britain's richest man, is the only confirmed bidder in the process, which was triggered by the Glazers announcing last November they were seeking a full or partial sale of the club or a possible strategic partnership with third parties.

Separate sources familiar with the bidding process have told ESPN that at least two groups with an interest in an involvement in United are sceptical that the Glazers are looking to sell all of their stake in the club, which they bought in 2005 following a leveraged takeover.

Multiple sources told ESPN that Joel and Avram Glazer, the United co-chairmen, have not ruled out the prospect of attracting a wealthy investor to help fund the club's plans to upgrade Old Trafford and United's Carrington training ground.

With the club's debt rising to £514.9 million in their most recently published accounts last November, and more than £300m owed in transfer payment instalments, sources have said the Glazers require outside investment to ensure that the club can compete at the top level of the game in England and Europe.

But with four of the Glazer siblings having already reduced their shareholding in recent months, sources have said that Joel and Avram accept that they cannot hold onto United unless they find outside investment through the sale process.

Sources have told ESPN that the Glazers value the club in the region of £6 billion -- substantially higher than the £3.8bn world record sale price for sporting team of NFL franchise Denver Broncos last year.

Premier League rivals Chelsea were sold for £2.5bn, with a further £1.75bn committed to future investment, to a US-based consortium last May.

Sources have said that the £6bn valuation is unrealistically high due to United's debt and need for additional funds to upgrade Old Trafford.

But with a minority, or 50% stake, proving more affordable, the Glazers could yet remain at Old Trafford by attracting a financially-powerful partner.