The statement was made in the prospectus for BT's forthcoming rights issue and adds to doubts over the value of the £10bn the company paid for 3G licences last year.
"Developing 3G technology may take longer than BT anticipates and prove not be superior to existing [GSM] technologies," the prospectus warned.
It also declared that building the network infrastructure to support 3G "will require a further investment of up to £10bn over a five-year period."
The company said it still planned to make this substantial investment but added the proviso: "The size of the [3G] market is as yet unknown and may fall short of expectations. BT cannot be certain that the demand for such services will justify the related costs."
The prospectus advised shareholders that the company could not give "any assurance that BT will make an economic return from its investment in 3G licences or networks. In the event that BT fails to generate significant revenue from its planned 3G mobile service offering, it may not be able to meet financial obligations incurred in relation to the 3G network or otherwise and its business, financial conditions and results of operation may suffer."
Eric Berg, editor in chief of the PricewaterhouseCoopers (PwC) 12th annual Technology Report, this week offered a more upbeat assessment of 3G's future, emphasising that it is "still a valuable long term investment".
Over the next two to three years 2.5G will be the most important technology, according to Berg. "The move to 2.5G is a much easier transition for the mobile service operators, with new 2.5G services providing a method to fund 3G expansion," he said.
European carriers have spent in excess of £100bn on 3G licences and the PwC report expects very little return on that investment in the next three years.
The BT shareholder prospectus is at http://www.groupbt.com/investorcentre/ .