Post-mortem tests carried out by a government laboratory show pigeons in Trafalgar Square are starving to death, according to a campaign group.
It is illegal to feed birds in Trafalgar Square
The Veterinary Laboratories Agency examined five corpses and found them to be in "poor bodily condition" and the "gizzard of all birds was empty".
Pigeon Action Group (PAG) says a ban on feeding the birds, introduced by mayor Ken Livingstone in 2003, is to blame.
The mayor's office said measures to reduce pigeon numbers were humane.
It became illegal to feed birds in the main square in 2003, but a loophole allowed protesters to continue to feed pigeons and other birds on the north terrace.
This loophole was closed by Westminster Council in September and pigeon numbers are thought to have dropped to between 300 and 400.
PAG claims that in 2002 a flock of about 4,500 birds occupied the square.
In October PAG sent five dead pigeons found in the square to the Veterinary Laboratories Agency's (VLA) laboratories in Suffolk.
The autopsy report said: "All were found to be in poor bodily condition and although in some of the birds there was evidence of recent feeding... the gizzard of all birds was empty."
The tests found no evidence of salmonella or the notifiable disease paramyxovirus.
Julia Fletcher of PAG said: "While the mayor is reporting that the programme to reduce the birds is working well, the reality is that these birds have been subjected to a long, agonising death by starvation."
She said that due to the birds' homing instinct they will not go elsewhere for food and the group wants a designated feeding area set up in the square.
A spokesman for the Greater London Authority said: "The GLA measures to reduce the feral pigeon population in Trafalgar Square and make the square more pleasant and hygienic for public use have been done in a humane way.
"It considers that it has at all times acted responsibly and fairly."