Sir Paul McCartney has written a letter to NASA urging them to reconsider using monkeys as part of their research into space.
The US space program plans to expose the animals to radiation experiments equivalent to three years in space.
But Sir Paul, a keen animal rights campaigner and vegetarian, said the organisation should seek alternative testing methods.
“I believe NASA has the ingenuity to investigate the health effects of space travel without confining and experimenting on animals as was done in the old days,” Sir Paul wrote in the letter, posted on PETA.org.
“It would be terribly disappointing if in our zeal to explore new frontiers and to learn about the fascinating universe where we live we began to regress in our treatment of the animals with whom we share this planet."
Before human space travels, monkeys were sent into orbit so scientists could see the effects of space travel on the body.
Sir Paul McCartney - Live