The carrion-eating bird became extinct in England and Scotland at the end of the last century because of persecution by gamekeepers and farmers, with only a small population clinging on in Wales.
Four of the dead birds were found in southern England, near where most of the imported birds have been set free. The other was found in the Midlands, where nine young kites were released last summer in an attempt to establish a second breeding population in England.
One of the birds was found dead on its nest. All are thought to have eaten bait left out in the open and laced with alpha chloralose, a rodenticide.
However, the bait was probably intended to kill foxes or ravens rather than the red kite. The practice of setting out poisoned meat in the countryside is illegal but still widespread.
Barbara Young, chief executive of the Royal Society for the Protection of Birds which has been sponsoring the reintroductions, said: "I'm appalled by the thoughtless and illegal use of poisons which is jeopardising the success of this highly important reintroduction project."
Since 1989, almost 200 young red kites have been released in southern England and northern Scotland in a joint programme between the society and government wildlife conservation organisations.
The new arrivals have begun breeding in both countries and their population is building up. But poisoned bait had killed birds before this year, and in 1995 another was found to have been wounded by shotgun pellets.