A raid which resulted in the deaths of a British and and Italian hostage in Nigeria represented the "best chance" to rescue the pair alive, MPs have been told.
Defence Secretary Philip Hammond told the Commons the Government moved to support Nigerian troops in the attempted rescue of Chris McManus and his Italian co-worker Franco Lamolinara after receiving credible information of their location and "imminent and escalating" threats to their lives.
In response, shadow defence secretary Jim Murphy endorsed the Government's decision to launch the raid last Thursday.
In a statement to MPs, Mr Hammond said: "The assessment on the ground was there was a significant possibility the kidnappers, if present, were already aware their security had been compromised and if they were not, the level of military activity in the town meant there was a real risk of them developing that awareness.
"The military judgment was the hostages were facing an imminent and escalating threat and while an immediate rescue attempt would inevitably involve risk it represented the best chance of securing the release of Chris and Franco alive."
Mr Hammond said Prime Minister David Cameron gave the go-ahead for British involvement in the assault on a house in the north-western town of Sokoto after a briefing by military and national security advisers.
He told MPs the operation lasted about 90 minutes but that the bodies of the two hostages were found by the troops in a room at the rear of the compound.
The Defence Secretary added: "The early indications are clear both men were murdered by their captors with automatic gun fire before they could be rescued."
Mr Hammond told MPs a coroner's inquest into the death of Mr McManus would be held.
Mr Hammond told MPs the Cobra emergency committee had met regularly during the 10-month captivity of Mr McManus and Mr Lamolinara to receive updates on the situation.
He said it continued to be the policy of the Government to not pay ransoms to terrorist groups who take hostages.
Mr Hammond said it had become clear following the kidnap that it had been carried out by the terrorist group Boko Haram and clear demands had not been issued.
He told the Commons some direct contact was made to Mr McManus's family.
In his statement, Mr Hammond said: "Where terrorists are involved in hostage taking, payment of ransoms is illegal under UK law.
"Over the course of Chris and Franco's captivity the Government's emergency committee Cobra met regularly to review progress and to discuss steps to secure their safe release. During their captivity the kidnappers made threats through a video and by direct contact with Chris's family they were intending to kill Chris and Franco.
"But at no time during their captivity did the kidnappers make any coherent demands."
Mr Hammond said the British and Nigerian Government had worked closely together to try and establish where the hostages were being held.
He said on a visit to Nigeria in July 2011, Mr Cameron had agreed a package of support from Britain for Nigeria's counter-terrorism efforts with President Goodluck Jonathan.
Mr Hammond said: "As part of that package a sustained operation was conducted to identify members of the group responsible for the kidnapping.
"Earlier last week, a number of them were apprehended and during de-briefing late on March 7, credible intelligence was obtained identifying the probable location of the hostages at a house or compound in Sokoto, northern Nigeria."
Mr Hammond said Foreign Secretary William Hague then briefed Mr Cameron that evening before chairing a Cobra meeting on the new information the following morning at 8.15am. A further full briefing was then relayed to the Prime Minister.
The location was confirmed and the Nigerian military took up position on the ground and an assault group including UK support was put in place.
Following a further briefing, the Prime Minister gave authorisation to the rescue attempt, which began at 10.58am London time.
The UK's ambassador in Rome informed the Italian government an operation was beginning "as soon as possible afterwards", Mr Hammond said.
Mr Hammond told the Commons that early in the approximately 90 minute operation, British forces killed one of the hostage takers. Three more were killed by Nigerian forces later.
One Nigerian soldier was wounded in the engagement, he said. Mr Hammond said he wished to pass "gratitude" to both the Nigerian forces and UK troops involved in the rescue attempt.
The Defence Secretary said: "This was a difficult operation that it was judged had to be carried out at speed in view of the risks to the lives of Chris and Franco.
"The deaths of Chris McManus and Franco Lamolinara were a terrible tragedy but let us be clear the responsibility for their deaths lies squarely with the people who kidnapped them, held them, threatened them and then murdered them in cold blood.
"Terrorism and kidnapping can never be justified. Many of the group responsible for the kidnapping and murder of Chris and Franco, including their senior leaders, are either dead or have been detained - an important achievement in reducing the threat of future kidnapping.
"But violent, extremist Islamist groups remain active in Nigeria and so long as they are we will work with the Nigerian and other allies to fight the scourge of terrorism wherever it manifests itself."