MOBILE phone group mmO2 reported losses of £873m, in its first set of results since demerger from BT last autumn.
The group, which is expected to break into profit in five to six years, said the market remained challenging, with subscriber growth slowing.
MmO2 owns UK mobile business BT Cellnet, which is being rebranded as O2.
The figures come a day after rival Vodafone announced the UK's largest loss, at £13.5bn.
MmO2's losses for the year up to March 31 included charges of £27m relating to the demerger and £123m of restructuring costs, which included cutting 1,400 UK jobs.
Figures were also obscured by a £369m write-down of the book value of previous acquisitions, seen as a routine accounting charge, and hit by £25m of interest charges.
It compares with last year's £3.5bn losses, when the group was part of BT's Wireless division and was hit by writing- down the value of some of its assets.
Chairman David Varney said: "mmO2 has delivered results in line with expectations at the time of the demerger, despite a challenging industry environment.
"Our market environment remains challenging, with subscriber growth slowing and the market for mobile data services still in its early stage of development."
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