The raid resulted in the deaths of a Briton and an Italian held hostage by a militant Islamist group.
Chris McManus and Italian Franco Lamolinara had been kidnapped last May while working for a construction company in northwest Nigeria.
British Prime Minister David Cameron said they were killed by their captors in a rescue mission involving Nigerian and British special forces.
President Giorgio Napolitano led a chorus of Italian condemnation on Friday, saying: “The behaviour of the British government in not informing Italy is inexplicable.”
“A political and diplomatic clarification is necessary.”
British Defence Secretary Philip Hammond told the BBC that there was a narrow window of opportunity to try to free the hostages because intelligence showed that they were about to be moved and possibly executed.
“It’s very unfortunate, but it’s completely explicable.”
Asked whether the Italians had approved the operation, Hammond said: “They were informed of it. I don’t think they specifically approved it, they were informed of what was happening.”
Prime Minister Mario Monti said Italy had been informed only after the raid began against a compound in the town of Sokoto. The British government confirmed this on Friday.
“Italy wasn’t informed or asked its opinion about a blitz that put at mortal risk an Italian citizen,” Fabrizio Cicchitto, a senior official in former Prime Minister Silvio Berlusconi’s People of Liberty party, said in a television interview.
“Between allies, this sort of mission is usually talked about beforehand. The British government bypassed and completely ignored us,” he said.
While Italian media criticised Britain for acting unilaterally, commentators also said the event underscored Italy’s diminishing international clout.
They linked the incident to an ongoing struggle by Italy to free two marines on anti-piracy duty who are being held in India for shooting dead two fishermen in the Indian Ocean.
“The United Kingdom still acts, maybe unconsciously, with the nostalgia of imperial glory,” said Antonio Puri Purini in Corriere della Sera, the country’s biggest daily.
BRITAIN DOWNPLAYS
The British ambassador in Rome visited the Italian Foreign Ministry “on his own accord” on Thursday night, a British Foreign Office spokeswoman said without giving further details.
British Foreign Secretary William Hague, speaking in Copenhagen before a meeting of EU foreign ministers, said he planned to speak to his Italian counterpart Giulio Terzi about the raid.
Hague played down the spat, saying “everybody understands the constraints involved, the rapid timing involved in a case like this.”
A Downing Street spokesman said Britain had been in close contact with the Italian government since the kidnapping last May. Rome was contacted as the operation got underway, he said.
“The fact of the matter is things were moving quite quickly on the ground and we had to respond to that and our top priority was to maximize the chances of getting the hostages out.”
Asked if Italy’s premier had given prior approval to a rescue operation, he said: “When the prime minister (Cameron) phoned Mario Monti, the operation had happened. We knew that the hostages were dead.”
Monti also spoke to Nigerian President Goodluck Jonathan, whose special forces made up most of the attack force, on Thursday to demand a “complete reconstruction” of the operation.
The hostage takers were a faction of militant Islamist sect Boko Haram that has links with al Qaeda’s north African wing, a senior official at Nigeria’s State Security Service said.
Boko Haram is waging an insurgency against Nigeria’s southern dominated government and has been blamed for shootings and bombings that have killed hundreds in the last two years.
The two diplomatic incidents in Nigeria and India are an unexpected challenge for Monti, who has focused primarily on economic reforms.
He took power at the head of an unelected government of technocrats in November, replacing the scandal-plagued Berlusconi as Italy teetered on the brink of ruinous default.
Despite being a NATO member and active in international peacekeeping – with troops in Afghanistan, Kosovo, Lebanon and elsewhere – Italy’s international influence seems to have flagged in recent years.
Berlusconi’s flamboyant personality, sexual and corruption scandals and diplomatic gaffes damaged Italy’s reputation abroad, especially after his foot-dragging when Britain and France pushed for the NATO bombing campaign that helped oust Libya’s Muammar Gaddafi.