British Prime Minister Theresa May will pledge to send 440 extra troops to Afghanistan during the Nato summit in Brussels on Wednesday as US President Donald Trump urges allies to contribute more.
The extra troops will be from the Welsh Guards regiment, with around half deploying in August and a second contingent to follow in February.
"In committing additional troops to the Train Advise Assist operation in Afghanistan we have underlined once again that when Nato calls the UK is among the first to answer," May is expected to say at the summit.
The troops will help "bring the stability and security that the Afghan people deserve," she is expected to say.
The extra deployment will take the total number of British troops in the country to around 1,100.
The last UK combat troops left Afghanistan in 2014 after being involved in the conflict since 2001.
Nato countries have a total of 16,000 personnel in Afghanistan to support Afghan forces, including 13,000 US troops.
Nato asked Britain to deploy more forces last year and Trump has repeatedly called on Nato allies to increase their defence spending.
Britain is one of just five Nato members to meet the target of spending at least 2.0% of GDP on defence.
"The alliance can rely on the UK to lead by example, not just in meeting the 2.0% pledge but by contributing our cutting edge capabilities to operations around the world," May will say.