The focus of the 4,100 troops still in southern Iraq was now on completing the task of training and mentoring the 14th Division of the Iraqi Army in Basra.
"As we complete these tasks and as progress continues across these different areas, we will continue to reduce the number of British troops in Iraq," he said.
"We would expect another fundamental change of mission in the first few months of 2009 as we make the transition to a long-term bilateral relationship with Iraq."
He urged Syria to "clamp down on the movement of foreign fighters" and warned Iran to "stop the provision of arms and training to those who attack a democratically-elected government in Iraq or the coalition forces in Iraq and the Iraqi people".
Mr Cameron praised the "incredible job" carried out in "difficult circumstances" by British troops, but he said he would judge the prime minister by his action, not his words.
"Clearly, everyone wants to see our forces withdrawn from Iraq as soon as it is practical to do so," he said.
"But do you agree with me that looking back over the last year, there are two important lessons to learn? The first is that we shouldn't make premature announcements about troops withdrawals which cannot then be delivered."
Earlier, the Commons defence committee said the security situation in the country had been "transformed".
But UK training of Iraqi forces in Basra must be a "medium-to-long-term project", it said.
Maintaining a sizeable training commitment was important to ensuring Britain remained an influential player in Iraq, as the country - potentially one of the biggest oil-producers in the Middle East - recovered its power and prosperity.
It said the success of an anti-militia operation, largely carried out by the Iraqi Security Forces (ISF) with British and US support, had led to a "seismic shift" in the balance of power in the city.
Last year, all training had been carried out at the main UK base at Basra Airport because it had been too dangerous to operate outside, but now the teams were with their Iraqi "parent" units around the region.
The committee said that it was "vital" to the stability of southern Iraq that their work was able to continue, even as overall British force levels were reduced.
The removal of the militia from their malign influence had made a complete change to the prospects in Basra and we were very encouraged
James Arbuthnot MP
The committee said that, while there was now a high degree of security in Basra, some areas of the city were not yet fully under the control of the ISF.
"There is no doubt that more remains to be done. The UK government must ensure that it continues to provide support to the ISF to ensure that the progress which has been made is not lost and that Basra does not slip back into instability," the committee warned.
It said Iran - previously accused by Britain of supplying sophisticated explosive devices to the Iraqi militias - could continue to cause trouble in the country.
Conservative MP James Arbuthnot, who led a committee visit to Basra this year, said the city had been "completely transformed" in the last 12 months.
"Last year when we were there, there seemed to be rocket and mortar attacks coming in every couple of hours or so. This year, we were there for five days - not a single rocket or mortar attack," he told BBC Radio Five Live.
"The removal of the militia from their malign influence in Basra in March and April by Prime Minister Maliki had made a complete change to the prospects in Basra and we were very encouraged."
Retired Maj Gen Tim Cross, who was the most senior UK officer involved in post-war planning in Iraq, warned: "Events will drive us, as they always do.
"The situation on the ground will drive us, and there will be, I am sure, many ups and downs in the next few months."
He told BBC Radio 4's Today programme: "Getting to the point where you can hand over responsibility to the in-place Iraqi forces is key to us being able to move on.
"We need to work alongside them, generally speaking, hand over progressively. The intent of handing over Basra airport will be the culmination of that."
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