The six-year British military mission in Iraq officially ended in April
All remaining British troops in Iraq have been relocated to Kuwait because the Iraqi government has yet to extend their right to remain.
A mandate allowing UK forces to stay in Iraq expires on 31 July and about 150 personnel have been moved out until an agreement is reached.
The mandate has not yet passed its third reading in the Iraqi parliament.
Most UK troops have already been withdrawn from Iraq but some remained to train the new Iraqi navy.
The remaining troops were mainly Royal Navy personnel.
The majority of UK forces pulled out in April but an agreement between the UK and Iraq governments to allow some personnel to remain was signed by Iraqi ministers on 6 June.
The UK government is discussing the position with Iraqi authorities, the Ministry of Defence said.
Defence Secretary Bob Ainsworth revealed British troops could be out of Iraq until late September, due to its parliamentary recess and religious fasting period of Ramadan.
In a letter, dated 24 July, to his Conservative shadow counterpart, Liam Fox, Mr Ainsworth said the government had been "deliberately keeping a low public profile" on the issue so as not to increase the risk to UK forces.
He told Dr Fox: "(The agreement) had successful first and second readings but the imminent Iraqi summer recess and Kurdish elections on July 25 have meant that their parliament has not yet been able to reach the quorum needed to have the third and final reading.
"The recess in August and Ramadan (which lasts from August 20 to September 20) means the agreement may not now be ratified until late September."
An MoD spokesman said US troops would be standing in for British troops while they were out of the country.
"While this delay is unfortunate, we are continuing to seek a solution with the Iraqi government that will provide our forces with the sound legal basis they need," he said.
"We must respect the Iraqi democratic processes."