Prime Minister Theresa May announced the government will send an extra 440 troops, which would bring Britain's total to about 1,100, to help Afghan troops fighting Taliban and Daesh (also known as ISIL or ISIS) insurgents.
The extra troops will be taking part in a NATO-led training mission, called Resolute Support, to train and assist Afghan forces. They will be based in Kabul and will not be in a combat role. British troops ended combat operations in 2014.
The announcement comes the day before a NATO summit in Belgium that could turn contentious over US President Donald Trump's insistence that allies pay more for their defense, Reuters reported.
Trump, who announced the United States would send thousands more troops to Afghanistan last year, has asked Britain and other NATO countries to send more reinforcements to the country.
"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 said.
"NATO is as vital today as it ever has been and our commitment to it remains steadfast. The Alliance can rely on the UK to lead by example."
The increase in British troops comes ahead of parliamentary elections in Afghanistan in October.
The extra British troops will initially come from the Welsh Guards, with around half arriving in August and the rest in February next year.