The letter says: "'Dignity in dying' is a phrase in common parlance in many sections of the population, being used by patients worried about the care they will receive.
"For the Voluntary Euthanasia Society to seek a monopoly of a common English phrase in order to invest it with a totally different meaning is dishonest and will create confusion."
However, Mark Slattery, a spokesman for Dignity in Dying, dismissed the letter as "a PR stunt by a cluster of lobbyists from the opposite direction.
"The bottom line is that they think the name is very effective," he said.
"They don't like that, because they don't agree with us."
A spokeswoman for Radar, the pan-disability campaign organisation, said the phrase was misleading.
"It's holding out the idea that assisted dying or voluntary euthanasia offers people a choice," she said. "We're saying disabled people don't have a real choice because they don't have an equal right to independent living."
On Monday, Dignity in Dying announced three new celebrity patrons: the actress Zoe Wanamaker, polymath Sir Jonathan Miller and author Michael Holroyd.
- See Paper Round, page 23, and Letters, page 25.