Staff were left "entirely distraught" by the death of the tigress, called Melati, the zoo's manager said Monday, rejecting suggestions they had not waited long enough before allowing the animals into the same enclosure.
Zoo staff held Melati, 10, and Asim, 7, in adjacent enclosures for 10 days before allowing them on Friday to try and breed as part of a Europe-wide programme to help boost the numbers of the endangered animal.
"Several people from outside the zoo have remarked that 10 days seems fast to introduce tigers to each other," Kathryn England, chief operating officer of the Zoological Society of London (ZSL), told staff in a letter published on her blog.
"It's not -– it's wildly variable and depends entirely on careful observation of their behaviour.
"Conversely, it can be risky to leave tigers showing an interest in each other out of contact for too long, leading to a build-up of tension and frustration," she added.
The pair initially watched each other, before rearing up – "all quite a normal part of them testing their boundaries," explained England.
"But in the blink of an eye, with no obvious provocation, they turned on each other.
"Asim started to retreat and we were poised to close the gate when Melati lashed back out at him. He overpowered her in a split second."
Staff lit flares, sounded airhorns and set off fire extinguishers and hoses, but Melati, mother of three cubs, was fatally injured.
Asim had been brought in from Denmark for the breeding programme.
"Even with the benefit of hindsight I am confident we'd all make the same decision again," wrote England.
"Friday was one of the most difficult days of my long career working with animals.
"I have known Melati since I joined ZSL in 2013 and I was as enamoured with her as everyone else at the Zoo," she added.
Staff were left "numb with shock," explained the zoo boss.
"Our vets ran to Melati... and even these experienced professionals...were entirely distraught."
She hoped the publicity surrounding the incident would "trigger a greater understanding of the challenges faced by her species in the wild."
"The coming days will be tough as we come to terms with what's happened, but we will treasure our memories of this special tigress forever."