Speaking at the G7 summit in Germany, Prime Minister David Cameron described the fight against Islamist extremism as the "biggest challenge" facing Britain and the international community.

The deployment will bring the number of UK military personnel in Iraq to 275.

It comes after Prime Minister Haider al Abadi requested more help in the struggle against IS, which has seized large parts of Iraq and neighbouring Syria.

The majority of the personnel, around 100 or so, will train local forces in detecting and dealing with improvised explosive devices.

The other 25 will focus on logistical challenges such as bridge-building.

While the British mission has so far largely focused on the Kurdish capital of Irbil, the new troops will be stationed at bases around the country, including the capital Baghdad.

It is understood they will stay inside the bases and will not be deployed in the field.

Mr Cameron said: "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.

"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.

He added: "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 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."

The announcement comes after 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 UK spending on defence may drop below the NATO target of 2% of GDP.

Mr Cameron has so far declined to commit the Government to continue meeting this commitment beyond March 2016.

Speaking before the meeting, Mr Cameron said he would have no assurance to offer Mr Obama until this autumn's Spending Review.

When asked what he would tell Mr Obama, Mr Cameron said: "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."