Mobile phone users have been warned they could face reception problems because it is the peregrine falcon nesting season.
Vodafone said three pairs of the world’s fastest bird have been found beside masts across London, including a church, hospital and office building, and could be slowing down networks.
Phone masts are a popular nesting site for the bird of prey and the company will be unable to remove the nests until the chicks are hatched and leave, which could take two months.
It is a criminal offence under the Wildlife and Countryside Act of 1981 to disturb the birds while they are nesting.
Vodafone spokesman Simon Gordon said: “We apologise to any customers who experience a dip in service, but we have to respect the environment and the law.
“This is the first time to my knowledge we have had this in London, so for us it is unprecedented. They can sometimes nest on the mast itself – which is like a big metal climbing frame – or use the box underneath, where the computer stuff sits.”
Mr Gordon, who is himself a wildlife enthusiast and bird watcher, added: “At the moment, we are a year into a £200million upgrade works and we are going to individual sites to upgrade the equipment.
“If there is a peregrine falcon nesting there, we have to abide by the law and not approach the nest and call the experts to go onto the site to assess where they are nesting.”
n April 2013, Vodafone customers complained about poor reception in Southampton. Engineers called out to repair a faulty transmitter found a peregrine falcon nesting next to it.
The company had to tell its frustrated customers they were unable to repair the faulty mast until the chicks had left the nest.
Peregrine falcons, which can travel at speeds of up to 240mph, have seen a revival in recent years.
The birds nearly became extinct in the 1960s after their existence was threatened by pesticides, but laws controlling use of pesticides meant their numbers slowly recovered.
The Royal Society for the Protection of Birds has estimated there are 1,402 breeding pairs in the UK and there are currently thought to be 30 nesting pairs in the capital.