The rapid growth of the parakeet population may pose a threat to other birds, authorities have said.
Parakeet numbers could reach 50,000 by 2010
There are about 30,000 of the bright green rose-ringed parakeets in London, with more in Surrey and Kent, and their numbers could reach 50,000 by 2010.
Increasing flocks will compete with native birds, such as woodpeckers and robins, for food and nesting space.
The Royal Society for the Protection of Birds (RSPB) said it had not ruled out a cull as a "last resort".
The Department for Environment Food and Rural Affairs (Defra) has commissioned a study into the parakeet threat.
RSPB spokesman Tim Webb said: "People are starting to have concerns about the numbers.
"We are not sure what sort of numbers they would have to grow to before becoming a problem but we are aware that they are growing.
"We do see a cull as a last resort, something only to be considered if a native species were to be under threat."
The RSPB has said that there is a single roost of 6,000 birds in Esher, Surrey.
Natural England, the licensing body for bird culls, said they would usually be killed by shooting, or possibly trapping.
Originally from India, the rose-ringed parakeet has been in Britain for decades, although it is unclear how it was first introduced.