Downing Street has insisted it did previously raise a possible hostage rescue attempt with the Italian government amid growing criticism in Italy of a special-forces raid in that left one Italian and one British hostage dead.
Number 10's disclosure came as Italian politicians and newspapers accused Britain of giving "a slap in the face" by not informing it of the mission until it was already under way.
Chris McManus, 28, from Oldham, Greater Manchester, and Franco Lamolinara died in the rescue attempt by UK special forces and the Nigerian military on Thursday. They had been held hostage by Islamists for nine months.
The Italian president, Giorgio Napolitano, said it was "inexplicable" that the UK did not inform his government before launching the rescue attempt.
In London, the prime minister's spokesman confirmed that the Italian government had been told of the rescue attempt after it had been launched, but added that the possibility of a raid had been raised in previous discussions and that no objection had been voiced.
"We have had lots of contacts with the Italians over the last nine months and a rescue operation was one possible operation," he said. There was no objection to this possibility, he added.
Italian officials were informed after the mission was under way, but by this stage, there was no possibility of stopping the mission, which was led by the Nigerians, he said.
There was no time to ask the Italians for their input because of the risk that the hostage-takers might learn that their location was known to the authorities, he added.
"The reality was that it was a fast-moving situation on the ground and our priority was that we had to react to this situation and maximise the possibility of getting these people out.
"In any situation such as this we need to take advice from people on the ground. Their advice was to act and act quickly. And that offered the best chance of getting these people out.
"Our assessment of the situation has not changed since yesterday. Early indications are that both men were murdered by their captors before they could be rescued."
Around 20 meetings of Cobra had been called and there was close co-operation between the British, Italian and Nigerian governments, he said.
Downing Street has not had any complaints from the Italians, the spokesman said. The British and Italian authorities have been working together to ensure the repatriation of bodies, he added.
In Italy, however, there was a growing backlash against the rescue mission.
"What happened in Nigeria is extraordinarily grave because governments are usually informed if they have co-nationals among hostages – they are warned and consulted," said Fabrizio Cicchito, the leader of Silvio Berlusconi's Freedom People party in the lower house of the Italian parliament.
David Cameron telephoned the Italian prime minister, Mario Monti, only after the raid by British special forces in Nigeria had failed, prompting a curt note from Monti that the raid had been "initiated autonomously by Nigerian authorities with British support, informing Italian authorities only after the raid had begun".
A spokeswoman for Monti told the Guardian that the Italian PM had been first informed about the operation "as it was already under way on Thursday morning".
"A few hours later he was informed about the tragic end of the operation by David Cameron," she added.
Monti then telephoned the Nigerian president, Goodluck Jonathan, to demand a "detailed reconstruction" of events leading to the death of Lamolinara.
An emergency meeting of Italy's parliamentary oversight committee for the secret services was under way on Friday morning to consider Britain's decision to keep Italy in the dark about the raid.
The leading Italian newspaper Corriere della Sera was highly critical of the operation, stating in a front page editorial on Friday that it was "an unacceptable slap and excuses are not good enough" and that the affair was a "humiliation" for Italy.
The newspaper quoted an unnamed source in Nigeria claiming it was likely the two hostages had been mistakenly shot by the Nigerian rescuers who took part in the operation. "Often during these types of operation, the soldiers have shown they are trained to put down heavy fire, kill a few people and if all goes well just apologise. This news does not arrive in because it does not involve the kidnapping of foreigners."
Corriere della Sera contrasted Italy's habit of negotiating for the return of hostages – citing Italy's release of Taliban prisoners in Afghanistan in 2007 in return for the freeing of an Italian journalist – with the British and American policy of refusing talks.
When British and Italian special forces teamed up to free two Italian secret service agents taken prisoner in Afghanistan in 2007, one died during the operation.
Thursday's raid, the paper said, proved that Britain was motivated by "nostalgia for its imperial glory" that prompted it to act unilaterally. Its treatment of Italy showed it treated the country as "hardly reliable".
Monti's government has come under fire at home for failing to free two Italian marines held in custody in India on suspicion of mistakenly shooting and killing Indian fisherman from a ship they were guarding against pirates.
Italian politicians said the Nigerian raid proved Monti's cabinet of technical experts, summoned to the government after the collapse of Berlusconi's administration, was failing to establish influence in international diplomacy.
"Technical experts can be technical experts but cannot do politics," said Daniela Santanche, a former minister in Berlusconi's last government. "One thing is international relations, another thing is balancing budgets."