BT Cellnet attracted 301,000 new customers between April and June, compared with Orange's 830,000 new subscribers during the quarter.
Demotion will come as a blow to British Telecom, which is gearing up later this year to spin off its wireless arm, which includes the Cellnet business.
The wireless division also revealed on Monday that it had bolstered its demerger drive with the board appointments of Paul Myners, chairman of Gartmore Investment Management and Guardian Media Group, and Andrew Sukawaty, former president of US personal communications giant Sprint PCS.
Genie thrives
Monday's statement also revealed a 41% surge in registrations to Genie, its mobile internet portal.
And BT stressed that for the first time its mobile subscriber figures included, in the pre-pay market, only active customers, defined as users who have used the firm's network within the last three months.
"[BT] expects other networks will in due course apply and adjust customer numbers to set a welcome new industry standard," the briefing said.
The adjustment reflects a growing acknowledgement by operators that, thanks to the low costs of signing up to mobile phone packages in the UK, many older handsets are thrown away rather than traded in.
"The change in our reporting base is a logical and important step to help our stakeholders clearly understand the underlying trends both in our business and the wireless market as a whole," BT Wireless chief executive Peter Erskine said.
The firm did not give figures for unadjusted user numbers.
Growth slows
The BT figures add further weight to observations that the UK mobile phone market is nearing saturation, with an Oftel survey on Monday estimating that 70% of Britons own a handset.
The maturing of the market is prompting operators to divert resources from grabbing new subscribers to increasing revenue from existing users.
"Today's announcement... shows our commitment to higher quality and profitable customer growth," Mr Erskine said.
Fifth operator
Deutsche Telekom-owned One2One is the fourth-placed operator in the UK, boasting about 9 million customers at the end of March.
The advent of third-generation mobile phone services - offering high-speed internet access, video and networked games - will see the arrival in the market of a fifth operator.
Hutchison 3G UK, which won a licence at auction last year, plans to launch services in mid-2002.
The group's shareholders include Hong Kong-based Hutchison Whampoa, Dutch telecoms group KPN and Japanese mobiles giant NTT DoCoMo.
In the City, BT shares closed down 11.25p at 455.75p on Monday.