A ‘mouse city’ the size of the population of York was accidentally bred by scientists following a laboratory blunder, a new Home Office report has shown.

Researchers at an unnamed laboratory bred nearly 180,000 more mice for use in experiments than their license permitted, and carried out unauthorised experiments on them, yet received only a letter of reprimand.

The mistake is the most alarming of dozens of non-compliance cases by labs across Britain, which are highlighted in an official government report into animal testing.

Describing the breach, the report said that a licence had been granted to monitor 127,600 mice over the course of one project, but ‘this number was significantly exceeded by 179,546’.

Scientists working on the project claimed error happened after numbers of mice were recorded on different databases.

The report also showed that a primate was left overnight in a recovery incubator after surgery without food or water, while other rodents died when they were not fed properly, or suffocated when their cages were not fixed to air vents.

And due to a ‘miscommuncation’ 74 chicks died after being left for 65 hours without adequate humidity.