Shell has promised to reveal which 210 of its 1,100 petrol stations have mobile phone masts hidden inside the forecourt price signs.

The oil company, which has a deal with T-mobile, said the transmitters were safe but concerns have been expressed by campaigners.

The only clue to their existence is a small light at the top.

T-mobile spokeswoman Gill Kerr told BBC News Online the antennae ensured "the about 46 million people with a mobile in the UK get the best possible service".

They promised faithfully they would consult with residents before they put these masts up

Mast Sanity director Lisa Oldham

But the director of the Mast Sanity pressure group Lisa Oldham said operators were "making a mockery" of the planning process.

"They promised faithfully they would consult with residents before they put these masts up," she added.

Controversy often surrounds applications to site phone networks.

Mobile operators were recently barred from putting the masts close to schools in the UK; many parents had said they were worried about health and safety implications.

Shell UK retail director Mick McMahon said: "We have a legal obligation to inform the local authority.

"We also have to meet stringent EU and UK safety guidelines and satisfy the local petroleum officer."

Nobody legally has a say in whether they are there or not

Campaigner Nigel Bruen

But Nigel Bruen, 52, from Martley, Worcestershire, who is campaigning against the siting of a transmitter close to his home, said: "These masts do not have to go through the full development process.

"In instances like these, it is classified as a permitted development and nobody legally has a say in whether they are there or not - they will not even know."

Meanwhile fast-food chain McDonald's has revealed "small coated fins" fixed to the walls of 120 of its restaurants were transmitters.

A spokesman said they emitted radio waves 10 to 20 times weaker than a baby listening device.