Campaigners want apes to have a right to freedom from captivity
Spanish Socialist MPs hope to persuade parliament to back a landmark project seeking human-like rights for apes such as chimpanzees and orang-utans.
Campaigners say the intelligence and self-awareness shown by apes mean they deserve rights to life, freedom and protection from torture.
Parliament's support would not be law, but would mean a commitment to the work of the NGO, the Great Ape Project.
The proposal has raised eyebrows and opposition in many areas.
One Spanish archbishop described the idea as ridiculous.
But for conservationists and Green MPs behind the proposal, it is a serious issue.
Green MP Francisco Garrido, who proposed the motion, says these creatures "so close to humans" have until now been considered as "mere objects or play things".
"The great apes have been tortured, mistreated, enslaved and murdered," he says on his website. "The habitats where their live have been wiped out and, according to the UN, they are in serious danger of extinction."
His colleague, Green MP David Hammerstein, told the BBC's World Today programme: "They show a degree of intelligence and awareness and, indeed, self-awareness.
"Their social and emotional needs are at the same level as handicapped people, small children, elderly, mentally impaired people - and they all have rights."
He insisted that they were not asking that the apes be given "legal or human rights".
"What we are talking about is very basic legal protection of rights which will guarantee each chimpanzee, bonobo or orang-utan the opportunity to live out his or her life according to his or her best interest," he said.
The international Great Ape Project is supported by environmentalists, conservationists and scientists.
Some say humans are close to chimps, but not close enough
But Professor Steve Jones, of London University, says the idea is an "overstatement of what science, what biology can tell you".
"As most people know, chimpanzees share about 98% of our DNA, but bananas share about 50% of our DNA and we are not 98% chimpanzee or 50% banana, we are entirely human and unique in that respect," he said.
"It is simply a mistake to use an entirely human construct, which is rights, and apply it to an animal, which is not human. Rights come with responsibility and I've never seen a chimp being fined for stealing a plate of bananas."
Archbishop of Pamplona and Tudela Fernando Sebastian said he could not believe it was even being proposed.
"We don't give rights to some people - such as unborn children, human embryos, and we are going to give them to apes," he said.
But Peter Singer, founder of the Great Ape Project says it need not stop with apes.
"I do think it is possible that we might want to extend this to other animals, perhaps progressively, perhaps as we discover more about elephants and dolphins," he told the BBC.
"Or maybe even more familiar animals like dogs or pigs, we might think that we owe them more in terms of moral status than we are currently inclined to give them."