The abortive bid to rescue Briton Chris McManus and Italian Franco Lamolinara in Nigeria was the hostages' "best chance", Philip Hammond has told MPs.
A decision to act was taken because "there was a significant possibility" the kidnappers knew their location was known, the defence secretary told MPs.
But the pair were killed before they could be rescued.
Mr Hammond faced questions about the failure to inform the Italian government before the raid.
He told MPs that the UK government had kept in touch with the Italian government throughout the 10 months the two men, found dead during the raid, had been held.
The two countries' security services had also kept in close contact and the Italians had known of the general "direction of travel".
Pressed on when the Italian government was told about the operation taking place, Mr Hammond said he could not specify a time.
The urgency is evident from the fact that the raid took place in daylight. It was deemed too risky to wait even 12 hours for dark?
Rescue bid: How pushed for time?
He said that the go-ahead decision was taken at a Cobra meeting chaired by Foreign Secretary William Hague which ended just before 9am on Thursday, and the "responsible officials" then went away to contact the UK ambassador to Italy, who was then to go, as soon as possible, to inform the Italian foreign ministry.
The operation began at 10.58am. On Friday the Italian president said it was "inexplicable" that the British government had not told Rome about the rescue attempt until it had begun.
In his statement, Mr Hammond said by Thursday morning: "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 judgement 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."
'Vindictiveness of opponents'
He told MPs the operation had lasted about 90 minutes but that the bodies of the two hostages had been found by the troops in a room at the rear of the compound with "early indications clear both men were murdered by their captors with automatic gun fire before they could be rescued".
Mr Hammond said the UK and Nigerian government had worked closely together to try and establish where the hostages were being held - learning of the location after debriefing captured suspects last Wednesday.
He also rejected the idea that any ransom had been paid, saying that as far as he knew no demand had ever been made.
"The deaths of Chris McManus and Franco Lamolinara were a terrible tragedy. But let us be clear that 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 contained - and important achievement in reducing the threat of future kidnapping."
For Labour, shadow defence secretary Jim Murphy said: "We believe the government took the right course of action in seeking to rescue two innocent captives."
But he questioned the apparent failure to keep the Italians informed.
Mr Murphy also said: "This tragedy is another painful reminder the UK must retain the ability to act across the globe. It is also reflective of the vindictiveness of our opponents and the valour of our forces.
"Can you convey the appreciation of Parliament as a whole to the commanders of the Special Boat Service for their remarkable efforts and bravery."
Nigeria Says Hostages Died Before Failed Rescue Raid - New York Times
<img alt="" height="1" width="1" /> Nigeria Says Hostages Died Before Failed Rescue Raid
[SIZE=-1]New York Times[/SIZE]
[SIZE=-1]ABUJA, Nigeria (AP) ??? A Briton and an Italian who had been abducted by Islamist militants were executed before they could be rescued by Nigerian troops and Royal Marines, Nigerian officials said on Wednesday. Nigeria's secretive State Security Service ...[/SIZE]
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Have you read anything other than the headline? Not really it said I had to log in.
It was the SBS, which is made up almost entirely of Royal Marines. SBS and Marines are separate things, it is made up of ex-Royals.
Alright lads, we don't want to inflame a simple situation!
The details aren't essential here - it doesn't matter whether they still count as Royal Marines or whether they count as ex-RM. At the end of the day, if any of us one day are good/fortunate enough not only to be Royals but also to end up in UKSF, we'll still be proud Royal Marines underneath it all!