Despite reports of a number of technical hitches in the new service, One-2-One will be launched officially before the end of September.
Although the service will initially begin within the M25 area, Mercury is already acquiring sites for expansion in the Midlands. This will confound critics who have suggested that the company would never offer One-2-One outside the London area.
Potential customers who have been given free trials of the service have met with patchy results, including interrupted or failed calls. This could cause potential problems in the lucrative City of London market.
However, a Mercury source said that the technical problems had been almost entirely ironed out. 'We are just about ready to go,' he added.
The number of 'cell sites', which are crucial to making smooth calls, has not been changed from the original plan of about 300 in the M25 area.
By next April the area covered will extend well beyond the M25 to cover towns including Milton Keynes, Reading, Tunbridge Wells and Chelmsford.
No timetable has been settled for expansion into the regions. But Mercury's licence requires it to cover 90 per cent of the population by the end of the decade.
Mercury One-2-One is a digital service that is of better quality and more secure than existing analogue cellular services operated by Vodafone and Cellnet.
The call charges are around 40 per cent less than current cellular charges, although some analysts question whether they are low enough to make mobile telephones a mass market product.
Vodafone is poised to launch its own digital service which will also allow customers to use their telephone in Europe and be billed at home.