MILLIONS of households across Britain will be alerted to the risk of interference to their Freeview TV signal in the coming months as mobile phone companies switch on 4G masts.

An industry-funded body helping viewers affected by the roll-out has already sent postcards to addresses in London that are in the vicinity of a 4G mast, warning of a chance their Freeview service could be affected.

Homes in Glasgow and Edinburgh are among 20 million in 16 cities that are due to receive the technology.

The communications regulator Ofcom predicted in July last year that as many as 2.3 million households would lose some or all of their digital TV reception when the 4G networks are rolled out.

But they now say they have revised their estimate down to 90,000 following trials.

Warning postcards will be sent to all those could be affected. Industry-funded body At800 will also send some households a free filter, depending on how likely they are to experience issues.

It has asked those who have received a postcard to get in touch if they have had a good Freeview reception in the past "but experience disruption to [their] television picture over the next few months".

O2, 3 Mobile and Vodafone are all able to launch their new 4G services on the 800mhz band, allowing their customers superfast mobile access to the internet.

But there have been concerns its proximity to the 700mhz band occupied by Freeview could lead to some viewers losing sound, pictures or entire channels.

EE, the only operator to offer 4G so far, uses the 1800mhz band, leaving Freeview unaffected, although it also has the option of using 800mhz.

At800 said tests suggested fewer than 1% of households with Freeview as their main television service were expected to be affected nationally, corresponding to approximately 90,000 homes.

Spokesman Ben Roome said: "We are contacting many multiples more than could be affected by this issue because the issue affects households as much on what their individual television set-up as on their location. If you have a decent new aerial and maybe a more modern television you might not see an issue, but the house next door, which might have poorer cabling or more sensitive piece of equipment, might not.

"We want to make sure that as many people who could potentially have an issue know about us should they have a problem."

Asked about areas of Scotland that might be affected, he said: "I think it would be reasonable to infer that initially you are talking about major urban areas."

Postcards sent to London households let them know who to contact in the "unlikely event of disruption", adding that a number of masts had been activated in London without any problems.

However, an Ofcom study from last year has revealed there will be a "small number" of households for which the filters will not restore reception.

It said that for those households the Government has decided to offer a TV platform change, like cable or satellite, and of the estimated 3% of those who cannot access any alternatives, up to £10,000 per affected household would be made available to provide some form of service.

At800 has ordered "a few million" filters that home owners can fit to their aerials to fix problems with interference.

Some of the £180 million funding provided by the mobile phone operators for At800 is thought to have gone towards recruiting engineers and 100 call centre staff. The Government has pledged to underwrite any overspend.

An Ofcom spokesman said: "We're pleased that the initial results, based on a few transmitters so far, suggest a relatively small number of homes are likely to be affected by 4G interference."