A species of bee not seen in the UK for a quarter of a century is being reintroduced to the countryside.
The short-haired bumblebee was once widespread across the south of England but it vanished in 1988.
However, after a healthy stock of the bees was found in Sweden, conservationists were able to collect some to seed a new UK colony.
About 50 queen bumblebees are being released at the RSPB's Dungeness reserve in Kent.
Nikki Gammans, from the Short-haired Bumblebee Project, said: "Normally, extinction means a species is gone forever.
"But it is magnificent that we can bring back this bee species and give it a second chance here in the UK."
The loss of the short-haired bumblebee ( Bombus subterraneus) was caused by the dramatic decline of wildflower meadows that occurred after World War II as agriculture intensified to feed the growing population.
It is estimated that 97% of Britain's flower-rich grasslands, which the bees needed to forage and thrive, has vanished over the past 70 years.
But in southern Sweden, the species is doing much better as fewer people live there and farming practices are more bee-friendly.
Dr Gammans said: "The bee population in Sweden is expanding and growing whereas for everywhere else in Europe it has been contracting - it is either rare, threatened, or extinct like in the UK.
"So Sweden was really the only place we could go to collect the bees."
A team of conservationists, with the permission of the Swedish authorities, captured nearly 100 spring queens to bring back to the UK.
Before the release, the bees were put in quarantine for two weeks at Royal Holloway, University of London.
Dr Mark Brown, from the university's school of biological sciences, said: "We've screened for four different parasite species, which can all damage bees in different ways.
"The key reason why we are looking for them is we don't want to introduce populations of these parasites from Sweden into the UK. Those with the parasites haven't been released."
The preparations for the bees' arrival in Kent have also been extensive.
At the RSPB's reserve in Dungeness, the site for the bees' release, conservationists have spent the last three years preparing the land.
Martin Randall, the site manager at the reserve, said: "The most important thing we've had to do to get this ready for the bees is to encourage wild flowers, like clovers and vetches.
"So the first thing we did was to collect locally grown clover seed and spread it across the grasslands, and then we followed it up by grazing it sensitively with cattle and sheep."
The work is already helping other endangered bee species in the local area: the shrill carder bee, which was absent from Dungeness for 25 years, was recently found there.
Mr Randall said: "When you come here on a still day, this is just buzzing with bees, and we're hoping that the short-haired bumblebee will join that group."
Local farmers have also been involved in the project, which has been funded by Natural England, the RSPB, Hymettus and the Bumblebee Conservation Trust.
By leaving margins unfarmed at the edges of their fields, flower-rich, green "corridors" are created, which will help the bees to spread out across the area.
Plight of the bumblebee
This is the second attempt to release the short-haired bumblebee in the UK.
In 2009, Dr Gammans collected bees from New Zealand, which had been introduced there from the UK in 1895 to pollinate red clover.
But DNA tests found the colony lacked genetic diversity and many of the queens did not survive their hibernation once in the UK.
But the ecologist is much more optimistic about the success of the Swedish bees.
Dr Gammans said: "We think there will be a really good chance that it will establish, it will become self sustainable and spread."
She said she expected between 20-30% of the reintroduced queen bees would survive after their release and create nests.
"This is about the usual survival rate for queens. After that, we want to add further reintroductions to increase the genetic diversity and increase their chances," she explained.
The team hopes the return of this species could give a boost to bee conservation.
Over the past few decades, bumblebees have been in serious decline. As well as the loss of the short-haired bumblebee, another bee species - the Cullem's bumblebee ( Bombus cullumanus) - has also been declared extinct in the UK and others species are at risk of vanishing from the UK.
Conservationists warn that the loss of the bees and other insect pollinators would be disastrous.
With about 80% of Britain's plants reliant on insects for pollination, it has been estimated that these creatures contribute more than £400m a year to the UK economy.
Dr Pete Brotherton, head of biodiversity at Natural England, said: "We depend upon nature in so many ways, yet across England many species and habitats are in decline.
"These losses can be stopped - today is a fantastic example of what conservation organisations, the government and farmers can achieve when we work together.
"Exciting projects like this one are vital in helping to turn the tide on biodiversity loss."