Speaking at the G7 summit of major industrialised states in Germany, Mr Cameron described the fight against Islamist extremism as “the biggest challenge” currently faced by Britain and the international community.
The new deployment, in response to a request from Prime Minister Haider al-Abadi, will bring to 275 the number of UK military personnel in Iraq supporting the Baghdad government and Kurdish authorities in their struggle with IS, which has seized large swathes of the country and neighbouring Syria.
The bulk of the personnel - around 100 of those deployed - will train local forces in detecting and dealing with the roadside bombs and booby-trap explosives used by IS. Another 25 will focus on logistics challenges such as bridge-building.
While Britain’s mission in Iraq has so far been largely focused on the Kurdish capital Irbil, the new troops will be stationed at a number of bases around the country, including Baghdad. It is understood they will remain inside bases and will not be deployed in the field.
The announcement came as Mr Cameron met US President Barack Obama in the margins of the Schloss Elmau summit for one-on-one talks which were expected to focus on defence issues.
Washington is concerned that UK spending on defence may drop below the Nato target of 2% of GDP, with Defence Secretary Ashton Carter last week warning that such a cut would suggest Britain was “disengaged” and could no longer “punch above its weight” on the international scene. Noting that the UK was one of only a handful of Nato states to meet the target, Mr Carter said: “We need an engaged United Kingdom.”
Despite lecturing other leaders at last year’s Nato summit in Wales on the need to hit the 2% target, Mr Cameron has so far declined to commit the Government to continue meeting it beyond March 2016. And Chancellor George Osborne’s demand last week for a further £500 million in Ministry of Defence cuts has fuelled concerns on the Tory backbenches that the UK may drop below the totemic threshold next year.
Speaking ahead of his meeting with Mr Obama, Mr Cameron said he would have no assurance to offer the US President until this autumn’s Spending Review, when Mr Osborne will set out the Government’s plans for the three years after April 2016.
Asked what he would tell Mr Obama about Britain’s plans for future defence spending, Mr Cameron told reporters: “I’ll say exactly the same as what I’m saying now, which is we’ve kept our 2% promise - one of the few countries to do it - and we’re having a Spending Review in the autumn and we’ll announce the results at that time.”
Mr Cameron said most of the new troops he is sending to Iraq will be involved in operations against improvised explosive devices (IEDs). When withdrawing from occupied areas, Islamic State routinely leave booby-trap devices on the roads and in cars, homes and official buildings in order to render the area uninhabitable for local people and slow the advance of regime forces.
“I think the biggest challenge that we face in terms of the effect on Britain and the challenge in the world is fighting extremist Islamist terror, particularly obviously in Iraq and Syria, but more broadly,” said the Prime Minister.
“We’re already the second largest contributor in terms of air strikes in Iraq. We’re providing a whole host of support to the Syrian opposition. But I’ll be announcing today that we’re going to be increasing our training effort in Iraq with another up to 125 personnel specifically focused on counter-IED training for the Iraqi army.
“We already have quite a number of people in the KRG (Kurdistan Regional Government area), but these people will be helping more broadly in Iraq. It’s a particular request that the Abadi government has made. It’s a particular thing that we’ve been working with the Americans on. I think it’s the right thing for Britain to do.”
Downing Street said the new deployment would begin “shortly”. A spokesman added: “These troops will be behind the wire at all times. That is where the training will take place and they will not be accompanying the forces they are training out on patrols.”
Mr Abadi is attending the G7 as part of a group of leaders of “outreach states” from areas threatened by Islamist extremists, including Nigerian President Muhammadu Buhari, who is facing an uprising by Boko Haram militants.