LONDON --- British communications regulator Ofcom is heading for a showdown with the country's two largest and original mobile operators over plans to take back part of the radio spectrum the operators have used since the 1980s to provide mobile phone coverage, and sell it to their rivals for 3G services.

Ofcom announced the plans last September and set in motion a scenario that would see another spectrum auction as it revealed plans to open some of the 900 MHz bands to more efficient use and increasing competition in mobile services.

The regulator suggested that some spectrum currently used for 2G networks by Vodafone and 02 be freed up and used for higher data rate networks by other operators. The two operators would, Ofcom says, retain "most" of the spectrum for their own networks.

It maintained that reclaiming part of the old 2G (second-generation) spectrum and re-auctioning it to rivals Orange, T-Mobile and 3 will lead to cheaper services, better indoors mobile coverage, fast wireless broadband in rural areas, 10,000 fewer masts across the country, and £6 bn worth of benefits to the economy.

Now, in stark contrast, O2 estimates that implementing Ofcom's plan would cost both companies a total of £13 bn and require 7,000 new mobile phone masts across the country.

In its response to Ofcom's proposals, Vodafone has also accused the regulator of making its decision "based on a hypothetical and fanciful version of the future".

O2, meanwhile, contends that "if the proposals are implemented, they will be based on a flawed policy mechanism and will have the certainty of occasioning very substantial costs... and disruption to customers while conferring benefits which are poorly researched, inadequately supported by evidence, wholly speculative and, on a proper view, insufficient to justify the intervention".