Plans to site more phone masts on top of the BT telephone exchange in central Winchester has re-fuelled campaigners' concerns about possible health risks.

Planners last week approved applications from T-Mobile and O2 to put up further masts on top of the building in Upper Brook Street.

The decision means the equipment will join a cluster of masts already on the site, belonging to operators, Vodafone, O2 and Hutchison 3G.

The new equipment for both companies was given the go-ahead on the condition that an independent study of the emissions of the whole cluster was undertaken.

Campaigners are becoming increasingly concerned that planners are being put under pressure to allow extra installations on existing sites despite little investigation into the possible health risks from such large collections.

"It's an awful site," said stalwart campaigner, Karen Barratt, who has spearheaded the on-going protest over plans from Orange to site a mast in Byron Avenue.

"There's so much equipment up there already, having this kind of concentration in a densely populated area is bad."

"You see clusters near motorways and I'm just afraid the BT exchange is becoming like that."

Speaking about the independent study, a condition of the siting of the masts, Mrs Barratt said: "The declaration on emissions, known as the ICNIRP certificate (International Commission on Non-Ironising Radiation Protection) may sound impressive.

"But it is simply a bit of paper signed by the operator promising to comply. Councillors have to take it on trust."

Mrs Barratt said the guidelines to which ICNIRP adheres only addressed heat effects caused by the radiation and not interference by pulsing.

Charity worker, Nick Haysom, who works in nearby St George's Street, is concerned about the long term health implications.

He said: "The BT building is right in the middle of a densely populated residential and work area. No one knows what the potential dangers are from this technology. People are being used as guinea pigs.

"There are operators there with incompatible systems and we just don't know what the long-term effects will be. It may be nothing but we won't know for 20 years.

"Many people will remember the fuss over irradiated vegetables - should we not worry equally about irradiated human beings?"

A city council spokesman said the operators had to "supply an emissions report to ensure it met with ICNIRP standards."

"We don't have in-house experts so we have to take the information from the mast operators themselves."