He said of course there are lots of questions about the support systems bin Laden had in Pakistan.
The Prime Minister, who last year said that the country faced both ways on terrorism, said “everyone including the government” in Pakistan would need to explain how the world’s most wanted man came to be living with his family in a compound near an army base.
It came as Asif Ali Zardari, Pakistan’s president insisted his country had not harboured the terrorist. Instead he said Pakistan had helped America and the west over many years with intelligence that could have played an important role in Bin Laden’s killing.
But Mr Cameron said: “There are a lot of questions that need to be answered.”
The Prime Minister will today update MPs about the killing of bin Laden. He will address the Commons at 3.30pm.
However, the Prime Minister, careful to avoid causing offence in Pakistan weeks after he visited Islamabad to mend fences with the country, went on to say that he would continue to work closely with the “political leadership” of Pakistan in tackling terror.
Mr Cameron also did not rule out the possibility that the death of bin Laden could hasten the return of British troops from Afghanistan.
He told the BBC: “I don’t want British troops there any longer than one moment they need to be.”
He urged the Taliban leadership in Afghanistan to take this opportunity to break from al-Qaeda, denounce terror, acknowledge the constitution of Afghanistan and work for a peaceful solution in the war-torn country.
Last night the Government's Cobra emergencies committee met to be briefed the group on the details of the US operation and reiterated his praise for the professionalism of the American special forces team.
Earlier in the day Mr Cameron spoke by telephone with Mr Zadari and prime minister Yusuf Raza Gilani and with Afghan President Hamid Karzai.
"The Prime Minister made clear in the conversations that Britain would continue to work extremely closely with both Afghanistan and Pakistan to tackle the terrorist threat from al Qaida and from the Taliban," a No 10 spokesman said.
"The Prime Minister also underlined the importance of effective co-operation between Afghanistan and Pakistan against terrorism and extremism."
Mr Cameron said Britain must remain vigilant against potential reprisals following the killing of bin Laden.
In a televised reaction from his official country residence Chequers that bin Laden's death would be "welcomed right across our country".
"Of course, it does not mark the end of the threat we face from extremist terror. Indeed, we will have to be particularly vigilant in the weeks ahead. But it is, I believe, a massive step forward," he said.
British embassies have been asked to review their security measures, but the formal threat level at home was unchanged.
British Foreign Secretary William Hague said he expected heightened vigilance at posts abroad for "some time to come".
"There may be parts of al-Qaeda that will try to show that they are still in business in the coming weeks as indeed some of them are," Hague said.
The Foreign Office advised Britons overseas to avoid large crowds and public events, and Defence Secretary Liam Fox said he had directed his department to "maintain a high level of vigilance in all UK defence facilities at home and abroad".
Britain remains at its second-highest threat level of severe, meaning a militant attack is considered highly likely.