The terrorists who killed Chris McManus and his Italian co-worker Francesco Molinara had “standing orders” to shoot the hostages should they come under attack, according to Sky News.
The report follows the interrogation of five of the suspects captured during the botched raid in Sokoto, Nigeria on Thursday.
According to Sky News, one of the men has said: "We had a standing instruction to kill the hostages immediately we sight any security agents around the building.
The fallout from the failed hostage rescue bid in Nigeria was continuing today after William Hague insisted it had been impossible to inform the Italian authorities in advance.
The Foreign Secretary said there had been a "limited opportunity" for saving the two construction workers, whose lives had been in "imminent and growing danger".
Briton McManus and Italian Franco Lamolinara died on Thursday as Nigerian troops and UK Special Boat Service (SBS) commandos tried to end their nine months in captivity.
The bid to rescue the men was apparently brought forward because the kidnappers - believed to be members of a jihadi group associated with al Qaida - became aware that the net was closing around them.
There were reports of a fierce firefight after the house in the north-western town was surrounded.
Italian president Giorgio Napolitano said it was "inexplicable" that Downing Street had not alerted Rome to the plan in advance.
"The behaviour of the British Government in not informing Italy is inexplicable," he said. "A political and diplomatic clarification is necessary."
And diplomat Antonio Puri Purini said the events had been an "unacceptable slap in the face" for his countrymen.
Writing in the leading Italian newspaper Corriere della Sera, he claimed Britain's nostalgia for its imperialist days had led it to act alone.
However, Number 10 said contacts had taken place between the governments as the operation got under way, and David Cameron spoke to Italian prime minister Mario Monti by phone after it was learned that the hostages were dead.
"We had been in contact with the Italians on a regular basis over the past nine months," the spokesman said.
"An option was always a rescue operation. We have been keeping them informed throughout.
"Things moved quite quickly in recent days and we had to respond to that. The Prime Minister was asked for authorisation and gave that authorisation, but this was a Nigerian-led operation."
The governments later tried to cool tensions in a joint statement issued after Mr Hague met Italian counterpart Giulio Terzi di Sant'Agata at a wider gathering in Copenhagen.
It said the UK minister "made clear that there had been a limited opportunity to secure the release of the two hostages whose lives were in imminent and growing danger".
"Under these circumstances it was only possible to inform Italy once the operation was already getting under way," the statement added.
"Mr Terzi expressed deep sorrow and disappointment over the tragic outcome of the operation and both ministers agreed on the urgency of sharing full information to facilitate the reconstruction and understanding of these events.
"Both ministers conveyed their solidarity and sympathy for the families of Chris McManus and Franco Lamolinara.
"They reaffirmed that the UK and Italy will continue to work together closely in the fight against such horrific terrorism and hostage-taking."