46,000 UK troops were in Iraq at the height of the conflict
British troops in Iraq will be reduced only if "conditions allow", Defence Secretary Des Browne has told MPs.
About 4,000 UK soldiers are serving in Iraq and the plan had been to cut that number at the next rotation in May.
But Mr Browne said that while the situation on the ground was evolving "it remains prudent that we take time to fully consider further reductions".
Foreign Secretary David Miliband, who has been on a visit to Baghdad, said the "downward trend" would continue.
The next rotation of British troops will take place this May. The Ministry of Defence was hoping to be able to reduce force levels, but that is now looking unlikely.
Mr Browne said: "It remains our clear direction of travel and our plan to reduce our force levels, as and when conditions allow, from the current number of 4,000.
"But while the situation on the ground continues to evolve rapidly, and while military commanders continue to assess the changing environment in Basra, it remains prudent that we take time to fully consider further reductions."
British forces are focusing on training and mentoring a Basra-based Iraqi army division to develop an effective force which can stabilise the area without assistance.
The MoD welcomes the Iraqi government's willingness to take responsibility for delivering security in the Basra province.
However, Mr Browne admitted that the Iraqi army division was "still months away from becoming fully operational".
He said British forces would not "desert them", adding: "We will continue to ensure that our support is tailored as effectively as possible to meeting its needs."
Meanwhile Mr Miliband, who has been visiting Iraq, told the BBC he had seen a "new confidence" in Iraq but added "one should never be glib" as there were still major challenges.
He said as the Iraqi forces gained in strength and confidence, Britain could make further decisions about its troop deployment, but added: "The downward trend that they committed to last year, will continue this year."
When Operation Telic 12 begins in May, 7 Armoured Brigade will take over as the lead troop formation from 4 Mechanised Brigade.