The Alfie star has teamed up with officials at the British Union for the Abolition of Vivisection (BUAV) to urge bosses of make-up firms to stop conducting tests on laboratory animals.
Michelle Thew, Chief Executive of the BUAV, says, "We are delighted to have the support of Sienna Miller. It is totally unacceptable for animals to continue to suffer and die in the name of beauty.
"Please join Sienna and support our No Cruel Cosmetics campaign to end cosmetics testing on animals. Sign our petition to the European Parliament at www.NoCruelCosmetics.org."