The UK is sending 250 more military personnel to Iraq, almost doubling its presence in the country.
Most of them will be going to Al Asad airbase in Anbar province, western Iraq, 100 miles west of Baghdad.
They include 50 trainers, 90 soldiers to protect the base and 30 to set up a headquarters. About 80 engineers will work on infrastructure for six months.
About 300 British personnel are already in the country helping to train Iraqi and Kurdish forces.
British forces will not be there to fight and will be confined to the limits of the base.
In a written statement to Parliament, Defence Secretary Michael Fallon said the deployment would add to the UK's "significant contribution" to the campaign against so-called Islamic State (IS, also known as Daesh).
"Our strike aircraft have now conducted around 900 air strikes against Daesh targets in Iraq and Syria and our intelligence, surveillance and reconnaissance aircraft provide niche and highly-valued capabilities," he said.
"On the ground, our forces have helped to train more than 18,000 members of the Iraqi security forces, including Kurdish forces.
"As Iraqi forces continue to regain territory and begin preparatory operations to retake Mosul, it is important that the coalition continues to provide the support needed to allow them to make further progress."
BBC defence correspondent Jonathan Beale says that while the deployment has been planned for some time, Mr Fallon clearly hopes it will reassure nervous allies in the wake of the Brexit vote.
About 120,000 members of the British armed forces and civilians served in Iraq following the US-led invasion in 2003.
UK combat operations officially ended in April 2009. A total of 179 UK troops were killed.