A male tiger mauled his intended mate to death after they were put in the same enclosure.
Asim, a seven-year-old male Sumatran tiger, killed 10-year-old tigress Melati after being moved to London Zoo as part of a European-wide conservation breeding programme.
It was hoped that Asim, who came from a safari park in Denmark, would mate with Melati and the pair had been in adjoining pens for more than a week so they could slowly become acquainted with each other.
But he attacked her after they were put in the same 2,500sq metre enclosure.
Zoo staff desperately tried to distract Asim with loud noises, flares and alarms but he overpowered Melati and killed her.
A spokesman for London Zoo said staff were “devastated” and “heartbroken”.
He said: “As with all big cats, introductions, however carefully planned, are always considered to be high risk.
“This morning, the two tigers were in separate paddocks and the adjoining door was opened to allow them to meet. Asim approached Melati and, as expected by keepers, the two tigers were initially cautious.
“Their introduction began as predicted, but quickly escalated into a more aggressive interaction.
“Zoo staff immediately implemented their prepared response, using loud noises, flares and alarms to try and distract the pair, but Asim had already overpowered Melati.
“Zookeepers were eventually able to secure Asim in a separate paddock so that they could safely get to Melati where our vets confirmed that she had sadly died.”
The zoo announced Asim’s arrival – whose name means “protector” in Arabic – from Ree Park Safari in Denmark, on January 29.
Head tiger keeper Kathryn Sanders described him at the time as “a handsome, confident cat who is known for being very affectionate with the ladies in his life”.
“We’re hoping he’ll be the perfect mate for our beautiful Melati,” she said.
The Tiger Territory area was closed for the day after the death.
Sumatran tigers are described as critically endangered by the World Wildlife Federation, which estimates there are fewer than 400 in the wild.