The prime minister has indicated that more British troops will be brought home from Iraq early next year.
Gordon Brown used a Commons statement on Tuesday to report on his weekend trip to the Middle East.
Brown told troops during his visit to Basra on Saturday that "enormous progress" had been made in Iraq recently and repeated the message to MPs.
However, he refused to set out what he said would be an "artificial timetable" for troop drawdowns and said the military force in Iraq will remain at a strength of 4,100 "for the next few months".
Troops were reduced from around 5,500 in September last year to roughly 4,500 by the end of 2007 and subsequently to 4,100.
Reduction
He said defence secretary Des Browne had been right in April to "pause the further planned reduction" to support the Iraqi-led security initiatives.
"Since then we have responded to changing needs", including embedding troops in the Iraqi command and protecting key economic interests such as Basra airport, oil platforms and water, Brown told MPs.
But while troops will continue "at these remaining levels" for the next few months, the prime minister reported that British military chiefs believe that Iraqi forces will be able to "take over military command of Basra airport at the end of this year" with training also to "be complete around the turn of the year".
He added that "as we complete these tasks we will continue to reduce our troops on the ground".
And following the move from a combat role to overwatch, he added that there would be a "further fundamental change of mission in the first months of 2009 as we make the transition to a long-term bilateral relationship with Iraq".
'Right'
Brown said: "It is right that having successfully trained and mentored large numbers of the Iraqi forces, having successfully worked with the Iraqis on a new economic development strategy, we now complete the key tasks we have agreed with the Iraqi government:
"Training the 14th division of the Iraqi army in Basra, preparing Basra airport for transfer to Iraqi control, pushing forward economic development, providing the necessary support for provincial elections, honouring our obligations to the Iraqi people and at the same time, and all times, ensuring the safety of our armed forces."
Criticising inconsistencies between the statement and off the record briefings, Conservative leader David Cameron called for the government to be as "clear and transparent on troop numbers as possible".
And he said the arguments for delaying the start of an inquiry into the lessons learned from Iraq were now "getting weaker and weaker".
Liberal Democrat leader Nick Clegg said the continuing presence of British troops was "contributing increasingly little".
Report
The statement came as a Commons committee found that the situation in Iraq is a "world away" from a year ago.
The defence select committee said in a new report that the security situation in Basra has been transformed in the past six months and the UK is helping the Iraqi security forces become self-sufficient and effective.
But the MPs also said that Britain would need to retain a small training force in the city for the foreseeable future.
Committee chairman James Arbuthnot said: "We have visited Iraq regularly as part of our scrutiny of the UK deployment there, and this year the security situation in Basra is a world away from what we saw last year.
"The Iraqi security forces have restored law and order to many parts of the city and the UK is working with them now to safeguard that stability and develop their capabilities.
"That will need a continuing commitment from the UK to maintain a military training presence in Iraq."
