Not all the animals in Berlin zoo receive the star treatment lavished on Knut the polar bear. As visitors snapped the once-cuddly celebrity yesterday, the zoo came under fire for allegedly dispatching three elderly black Himalayan bears and a hippo to the slaughterhouse to make the most out of their skins and meat.

The claims, made by the Green politician Claudia Haemmerling, have caused uproar in Germany. “This is frightening,” said Beate Holzbach, who was leading a school group towards Knut's compound yesterday. “It's inhuman just to rip apart animals after they have become too old.”

The 35-year-old teacher was speaking against a backdrop of screeching monkeys and the melancholy roar of various big cats: it was as if the whole zoo was protesting.

According to Ms Haemmerling, a family of Himalayan black bears and a pygmy hippopotamus were sold in the 1990s to exotic animals dealers by the Tierpark, now the eastern branch of Berlin zoo. They apparently ended up in the Belgian town of Wortel, near Antwerp, which boasts a slaughterhouse but no zoo.

The politician also argues that the zoo cross-bred panthers and Java leopards, as well as selling tigers to China to be used in impotence cures.

The claims are partly documented - some of the paperwork can be seen on Ms Haemmerling's website - and the state prosecutor has begun inquiries to see if there is enough evidence to open a formal investigation.

Bernhard Blaszkiewitz, director of the zoo and the man who authorised the rescue of Knut from a negligent mother, furiously denies the accusations. “This is a combination of misunderstandings and nonsense,” he said.

Mr Blaszkiewitz, who was the boss of the sprawling, open-plan Tierpark at the time that the animals were supposedly sold, said that the hippo had been sold to a reputable dealer who passed it on to a zoo in Ghent, in Belgium. The zoo closed later and he was unaware of what happened to the hippo.

The Himalayan black bears were old animals but, again, were sold to an established dealer. Animals were sold to China but there was no question of Berlin zoo having supplied animals for use in the Chinese version of Viagra.

But the pressure was building on Mr Blaszkiewitz yesterday. “If there is clear evidence that the animals ended up in a slaughterhouse, then Blaszkiewitz will ultimately have to resign,” said Wolfgang Apel, president of the German Animal Protection League.

Stomach-curdling pictures released on the internet yesterday by animal rights activists showed a tiger, supposedly from a zoo in Guestrow, in eastern Germany, being sliced apart in the Wortel slaughterhouse in Belgium. “Did Berlin's zoo animals suffer a the same fate?” asked the front page of the Berlin tabloid BZ.

This was expected to stoke anger against the zoo director, who has been in the sights of the animal rights movement since he authorised the hand-rearing of Knut.

Some activists have argued that this was a violation of animal protection laws, which stipulate that animals should be brought up in a natural way, appropriate to their species. These principles have been anchored in German law since the days of the Nazis.

“Animal torture should be punished not because it upsets compassionate human feelings but because the animal as such deserves protection against abusive behaviour,” said Clemens Giese, a prominent lawyer, in 1939. But in the final days of the war Berliners eagerly pounced on zoo animals that had been killed in Allied bombardment or which had broken free from their cages in the chaos. This relish for exotic animals after years of rationing caused such shame it was barely mentioned in history books for decades.

This goes some way to explaining popular upset at the possibility that German zoos may have their shadowy side: disposing of animal surpluses by slaughtering and selling the meat, perhaps to specialist gourmets.

Last year it was revealed that Erfurt zoo employees had been killing animals illegally - mainly deer and goats - for sale to butchers. No exotic animals appear to have been killed but there was no veterinary supervision.

Animal rights campaigners argue that zoo animals stand a good chance of ending up on someone's plate as long as animal inventories are witheld from the public. The organisation People for Animal Rights, says that monkeys from at least one German zoo have been sold to a laboratory for experiments.