"We will not cut and run, and we will not leave the job half done," Reid told a press conference in London alongside visiting Iraqi Prime Minister Ibrahim al-Jaafari on Wednesday.
"We will stand by Iraq when times are tough and we will be a committed friend, not a fair weather friend."
The British aim was "working towards an orderly transition of enabling the Iraqi forces to lead on security", Reid said.
"That is a process, it is not an event, and it is a process that is on track."
"We do not want to be in Iraq any longer than is necessary, but the pace of transition will depend on how long the Iraqi government judge that our forces are needed," he said.
No harm to relations
In turn, al-Jaafari said clashes between British troops and Iraqi police in Basra would not undermine the relationship between the two countries.
On Monday, UK troops stormed a
police station in Basra
"At this time, where there are forces in Basra and all over Iraq, such things are expected to happen," al-Jaafari said after talks with Reid.
"As for us, it will not affect the relationship between Iraq and Britain, and we hope that together we will reach ... the truth of the matter."
Al-Jaafari said Iraq had made "great progress" in rooting out religious and political sectarianism within the security forces.
"Previous infiltrations which caused some catastrophes have been narrowed almost to nil," he said.
British newspapers have queried whether the country's 8000 or so forces should be removed from Iraq, where they are based around the southern city of Basra, following events earlier this week.
On Monday, British forces stormed a police station in Basra in search of two arrested comrades, later found and freed from a house where they had been taken from a police cell by militiamen.
"We will not cut and run, and we will not leave the job half done"
British Defence Secretary
That incident sparked fears that fighters have infiltrated Iraq's police force, something Iraq's national security adviser Muwaffaq al-Rubaie conceded later was the case.
It followed a riot in Basra in which demonstrators firebombed two British armoured cars, forcing troops to flee the vehicles engulfed in flames, dramatic pictures of which were splashed across many British newspapers the next day.