However, the company's sponsorship of the BT Cellnet Riverside Stadium will continue until 2005 as part of a 10-year contract.
The decision by the telecoms giant follows the rebranding of its wireless division to mmO2 from its former name, BT Wireless.
BT first began sponsoring Middlesbrough FC's shirts in 1995 when the team -- then still in the first division -- moved to its new Riverside Stadium.
The team has since won promotion into the Premiership and BT Cellnet renewed its contract in 1999.
Middlesbrough is currently seeking a new sponsor for its shirts, to begin when its agreement with BT Cellnet expires in May.
Peter Richardson, BT Cellnet marketing and sales director, said the company had gained great benefits from its "strong relationship with Middlesbrough FC".
"Since the mid 1990s, BT Cellnet's massive growth as a company has been a huge part of a tenfold increase in the number of people using a mobile phones, from only 4m people to today's 40m plus.
"Now we are poised for a similar market transition to a wealth of new data services and we will also be moving to adopt the new O2 brand in the UK. It is therefore timely for us to review our sponsorship strategy."
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