An Italian and a British hostage kidnapped in May in Nigeria have been killed by their captors during a joint raid by British and Nigerian forces in an operation to free them, Italy's government said.

British Prime Minister David Cameron called Italian Prime Minister Mario Monti to inform him of the "tragic conclusion" of the operation, a statement said.

The joint forces intervened to free Italian Franco Lamolino and Briton Christopher McManus because they were fearful that their lives were under threat.

News of the deaths was announced by Prime Minister David Cameron who authorised British forces to help raid the location where they were being held.

He said initial indications were that the contractor from north-west England, and an Italian colleague snatched by gunmen at the same time in May, were killed by their captors.

Nigerian President Goodluck Jonathan said the two men had been murdered by "their Boko Haram captors" and those responsible have been arrested.

Boko Haram is a militant Islamist group that has carried out a number of attacks in the country.

In a statement, the family of Mr McManus said: "We are of course devastated by the news of Chris' death which we received earlier today.

"During this ordeal we have relied heavily on the support of our family and friends which has never waned and has enabled us to get through the most difficult of times.

"We are also aware of the many people who were working to try and have Chris returned to our family, and his girlfriend. We would like to thank all of them for their efforts."

They said they were confident "everything that could be done was being done" during their 10-month ordeal and sent their condolences to relatives of the other dead hostage, Mr Lamolinara.