AVA- The commitment to Nato's mission in Afghanistan will take the total number of UK troops in the country to 1,090.
Around half of the troops will deploy from the Welsh Guards in August and the remainder will follow by February 2019.
They will help "bring the stability and security that the Afghan people deserve," Theresa May said.
At the height of the war, Nato had more than 130,000 troops from 50 nations in Afghanistan. The UK had 9,500 personnel and 137 bases in Helmand province alone.
As of last month, the number of Nato personnel stood at 16,000 from 39 countries, operating mainly from Kabul and focusing on training and assisting the army and police.
Defence Secretary Gavin Williamson - who wrote to Mrs May recommending more troops - said "whenever the call comes, the UK is always one of the first to step up and support our friends and allies across the globe".
Nato asked Britain to deploy more troops a year ago. In September of last year, the US committed an extra 3,000 troops to support Afghan forces.
Ahead of the summit, which begins on Wednesday, US president Donald Trump, hit out at his Nato allies for failing to spend enough on defence.
"The US is spending many times more than any other country in order to protect them," Mr Trump tweeted.
Wednesday 11 July 2018 18:37