Mobile phone giant Vodafone, which has sponsored Manchester United since 2000, is ending a four-year shirt deal with the Premier League club.
Vodafone signed a four-year deal with Man Utd in late 2003
The latest deal, worth about £9m ($15.5m) a year, began only last season and will now end in May 2006.
Vodafone said it now wants to focus on Champions League sponsorship.
Manchester United said it was in talks with "a number of world class" potential suitors and was confident of securing a new deal in coming months.
United commercial director Andy Anson said: "The Manchester United shirt is the most iconic in sport.
"The club feels that in the current market there is a genuine chance to attract significant additional investment."
When the latest £36m, four-year deal, was signed in December 2003, Vodafone said it would roll out a wide range of club content, including results, news, still and video images, ring tones and games, through its services.
Vodafone is instead to start sponsoring the Uefa Champions League from next summer, as both an official partner and the official mobile network of the competition.
The world's largest mobile phone company by revenue has said it will continue to work with Manchester United, but that when the Champions League opportunity arose it was "too good to let go".
'Alignment of interest'
In July this year, the Manchester United Supporters' Trust, which had been opposed to the sale of the club to US tycoon Malcolm Glazer, urged Vodafone to end its sponsorship deal with United as the club was a "tainted brand".
Tamoil - Juventus, £15m a year
BenQ - Real Madrid, £14m a year (from 2006)
T-Mobile - Bayern Munich, £11.4m a year
Samsung - Chelsea, £10m a year
At the time Vodafone said it did not see any reason why it should not continue to benefit from Manchester United links.
"Vodafone has been there long-term and may feel it has got what it can from the deal during United's more successful years," said Harry Philp, managing director of Hermes Sports Partners, which advises on football sponsorship and investment.
"From the club's point of view it is all part of the Glazer takeover, and saying they would look to renegotiate contracts.
"The announcement may well be an alignment of interest for both parties."
He added that United would now be under pressure to match the biggest sponsorship deals in Europe.
The biggest European shirt sponsorship at present is between energy giant Tamoil and Italian club Juventus, worth 110m euros (£75m) over five years from July 2005, working out at £15m a season.
From next summer Real Madrid will be collecting a cool £14m a season from Siemens' partner Taiwanese mobile company BenQ Mobile.
Bayern Munich collects £11.4m a year from its deal with T-Mobile.
The biggest current shirt sponsorship deal in English football is the £50m, five-year contract Chelsea have with Samsung.
Manchester United have only had one other shirt sponsorship deal in their history, with Japanese electrical firm Sharp on the players' chests for 18 years from 1982.