Mr Cameron said he would “continue to do everything we can to hunt the people down who are responsible for this and for other such terrorist outrages''.
Each of the families of those caught up in the crisis has been assigned a police liaison officer and is being kept fully updated of developments, he said.
BP, the joint operator of the gas plant, has also begun evacuating staff from Algeria, with 11 being flown back to London yesterday.
The Government is providing a back-up service to fill any “gaps in what BP are able to do” and all installations in Algeria have been placed on “high alert”.
Labour leader Ed Miliband said those behind the “appalling and tragic events” should know Britain and the international community “stand united in condemnation”.
US defence secretary Leon Panetta said in London today that here was “no justification for the kidnapping and murder of innocent people” in Algeria, and vowed the US government was “working around the clock to ensure the safe return of our citizens”.
Mr Cameron said the terror attck was a “well calculated” assault that is believed to have started with an attack on two buses travelling to the compound.
He told MPs: “According to the information we have from the Algerian authorities, the terrorists first attacked two buses en route to the In Amenas airfield before attacking the residential compound and the gas facility at the installation.
“It appears to have been a large, well-co-ordinated and heavily armed assault and it is probable that it had been pre-planned.
“Two of those travelling in the convoy to the airfield were, very sadly, killed, including one British national and his family were informed on Wednesday.