A sombre-looking Cameron warned his compatriots to expect "bad news" after Algerian forces launched an operation to free the hostages from Islamist militants, saying one Briton had already been killed when the site was stormed on Wednesday.
Algerian forces stormed the desert gas complex to free hundreds of hostages but 30, including several Westerners, were killed in the assault along with at least 11 of their Islamist captors, an Algerian security source told Reuters.
"The Algerian armed forces have now attacked this compound," Cameron told BBC TV. "It is a very dangerous, a very uncertain, a very fluid situation and I think we have to prepare ourselves for the possibility of bad news ahead."
Britain and Norway, whose oil firms BP and Statoil run the plant jointly with Algeria's state oil company, said they had been informed by the Algerian authorities that a military operation was under way.
However, Cameron seemed irked that he had not been informed that the military action would take place.
Military training mission
EU foreign ministers met yesterday (17 January) to discuss developments in Mali, but the meeting was largely overshadowed by the Algeria hostage crisis which broke on Wednesday.
The ministers decided to speed up the deployment of the military training mission - EUTM Mal - in Mali following months of fighting between government troops and rebels linked to an Islamic extremist group.
The Council also discussed other possible EU actions to aid Mali, such as financial and logistical assistance for the deployment of African-led International Support Mission to Mali and other direct support to the Malian government.
In the speech he postponed, Cameron had been expected to spell out his plans to renegotiate Britain's EU membership and to promise a rare referendum on any deal he struck. Some politicians said that move could redefine Britain's role in the world, alienate key allies and determine Cameron's own political fate.
Cameron had been planning to say that the EU faces three major challenges: the eurozone debt crisis, faltering competitiveness and declining public support, particularly in Britain.
"If we don't address these challenges, the danger is that Europe will fail and the British people will drift towards the exit," Cameron had been due to say, according to extracts of his postponed speech released by his office.
"I do not want that to happen. I want the European Union to be a success and I want a relationship between Britain and the EU that keeps us in it."
But, he intended to say there was growing frustration about the widening gap between the EU and its 500 million citizens and that the status quo was untenable.
Harsh austerity measures imposed in many European states are making the problem worse, Cameron planned to say.
"There is a growing frustration that the EU is seen as something that is done to people rather than acting on their behalf. And this is being intensified by the very solutions required to resolve the economic problems," a draft of the speech said.
"People are increasingly frustrated that decisions taken further and further away from them mean their living standards are slashed through enforced austerity or their taxes are used to bail out governments on the other side of the continent."
Cameron, who wants to stay inside the EU, has argued that the upheaval created by the eurozone crisis has given Britain a chance to renegotiate the terms of its membership of the bloc.
There was no mention in the advance extracts of a possible referendum on Britain's role in Europe.