A Briton and an Italian held hostage in Nigeria were killed by their captors before they could be freed by a rescue mission, Prime Minister David Cameron said on Thursday.

Cameron said he had authorised the failed operation, conducted with the Nigerian government, after being told the men's lives were in imminent and growing danger.

Cameron said Briton Chris McManus and Italian Franco Lamolinara had been taken hostage in Northern Nigeria in May 2011.

Preparations were made to mount an operation to attempt to rescue Chris and Franco. Together with the Nigerian government, today I authorised it to go ahead, with UK support, Cameron said.

It is with great regret that I have to say that both Chris and Franco have lost their lives.

We are still awaiting confirmation of the details, but the early indications are clear that both men were murdered by their captors, before they could be rescued, he said.

In August a video of the hostages surfaced in the Nigerian capital Abuja with the two men on their knees and blindfolded, with three men wearing turbans and holding guns and ammunition behind them.

The terrorists holding the two hostages made very clear threats to take their lives, including in a video that was posted on the internet, Cameron said.

We also had reason to believe that their lives were under imminent and growing danger, he added.

The Italian government said it had only been informed about the rescue bid after it had begun. It said Cameron had called Italian Prime Minister Mario Monti to inform him of the tragic conclusion of the operation.

(Reporting by Tim Castle and Peter Griffiths in London, and Steve Scherer in Rome)