Today the owner of Hamerton Zoo Park, near Peterborough, said that he was sickened because two of the marmosets died after they were recovered by the police because they had been so badly treated.

Furious Andrew Swales said he was appalled that anyone would steal rare and endangered creatures from safekeeping at the zoo. He said the monkeys had been treated appallingly and left in squalid conditions when they were found in a police raid in Staffordshire. One had a broken pelvis.

Mr Swales said security at the 20-acre site had now been stepped up because the thieves had managed to cut through fencing and removed windows during the night to get in.

Once inside, they used screwdrivers to dismantle tree nest boxes that the marmosets lived in before taking them away.

The burglary was carried out so professionally, that staff at the zoo did not even know they had been raided until hours after the crime had been committed in April of this year.

There seemed to be no leads as to where the monkeys had gone until Sussex Police recovered 15 monkeys during a search of two addresses in Tunstall, Staffordshire.

Officers found marmosets which had been taken from Hamerton Zoo as well as from Drusillas Zoo in Alfriston, east Sussex and zoos in Devon.

Police arrested two 35-year-old men from Tunstall, who are due to answer bail on November 17, and have arrested another man and a woman in their 30s from Sheffield, who have been bailed until October 17.

Mr Swales said he was still saddened by what had happened.

He said: “The whole thing has shocked me. It was very carefully targeted and we didn’t know what was happening. It was clearly planned and to order because they knew exactly where to go and how to get the animals out of their boxes.

“It was very thorough, very careful and very quiet and they left minimal damage. It only ever happened once before in the early ’90s so we didn’t imagine someone would steal from a zoo.”

Mr Swales lost a family of common marmosets, with two parents and six child monkeys, and another family of Geoffrey’s marmosets, with two parents and three young monkeys. Some of those stolen were as little as two weeks old.

They also took a pair of macaw parrots, which also come from South America.

Hamerton only got two of the monkeys back. Both died within days of returning to the zoo.

Mr Swales said it was difficult to prepare for such awful burglaries.

He said: “This is not something that happens very often at all, so it is difficult to get the infrastructure in place to prevent it.

“However we have completely reviewed and revamped the security procedure.