Twenty-seven voles booked into Wildwood captive breeding centre in February 2009 after being rescued by Thames Water before it started a four-year expansion at the works. They have since produced 50 babies. Hazel Ryan, Conservation Officer at the Wildwood Trust, said: "The water vole is Britain's most endangered mammal so we're glad Thames Water has taken time to transfer the Crossness voles to us for safe-keeping.”

During their time at Wildwood, the voles are living in favourable conditions with a good food source, access to clean water and out of harm’s way from predators. As a result they have produced healthy offspring, which will go on to breed again this summer.

They will be brought back to Crossness, one of Europe’s largest sewage works, in spring 2011 when their new and improved habitat is complete. Work to expand the site’s treatment capacity by 44 per cent will be completed in 2014.

The expansion includes building new reed ditches for the voles to live in at the Crossness Marshes nature reserve, next to the works.