Secret mobile phone masts have been hidden from view inside signs at six petrol stations in Croydon.

The T-Mobile transmitters are disguised in forecourt price boards at Shell garages for "aesthetic" purposes, infuriating campaigners who have accused companies of deliberately trying to conceal the masts to dodge public outcry.

The only clue of their existence is a small, light at the top of the boards.

Planning permission is not needed for masts less than 15m high or if they are housed inside petrol station price boards, but the local authority must be consulted before they are put up.

MAST Sanity, a national organisation campaigning against radiation-emitting masts on health grounds, believes beleaguered phone companies are taking advantage of a planning loophole to avoid consulting residents.

Lisa Oldham, director of MAST Sanity, said: "The fact that they seem to be deliberately hidden from public view is suspicious and suggests that these companies want to avoid a backlash from local residents."

Shell's financial deal with T-Mobile, allowing them to place transmitters inside pricing boards, was made public last year and it was forced to reveal which 210 of its 1,100 filling stations were harbouring hidden masts.

MAST Sanity published a full list of Shell locations on their website, including filling stations in Parkway, New Addington, Whitehorse Road, Thornton Heath, Kent Gate Way, Addington, Limpsfield Road, South Croydon, Coulsdon Road, Old Coulsdon and Portland Road, South Norwood.

Campaigner Gerry Meredith-Smith, from Sandilands in Addiscombe, believes the problem lies in the local planning procedure where the visual impact is the primary consideration.

He said: "There is not enough evidence to stand up health concerns but neither is there enough evidence to disprove it.

"The fear and concern that people experience is sufficient enough for it to become a health issue and should therefore become a planning consideration."

A Croydon Council spokesman confirmed that because health implications over masts have not been proven, it is not an issue when considering applications.

A spokesman from Shell UK said all of the 210 antennas in Shell sites are safe and were installed following consultation with local authorities.

Mick McMahon, UK Retail director said: "We fully recognise the public's concerns and we are working proactively with the mobile phone companies to ensure ongoing transparency."

A spokesman from T-Mobile insisted the reason behind concealing masts was for aesthetic purposes, not to avoid public protest, saying: "We do understand people's concerns but we have to meet the growing demand for customers as well as measuring the environmental impact."