The 100 troops deployed to Afghanistan form the first group of the additional 440 personnel expected to be sent for the Nato mission as part of a commitment made by UK Prime Minister Theresa May in July this year.
With the addition of 100 Welsh Guards, the country will become the third largest contributor to Afghanistan with approximately 1,100 personnel.
UK Defence Secretary Gavin Williamson said: “We remain absolutely committed to building an Afghanistan that is a strong security partner for decades to come. A secure and stable Afghanistan helps keep the streets of the UK safe.
“The uplift of UK personnel not only underlines our commitment to Afghanistan and regional security, but also to the Alliance that has been the cornerstone of our defence for 70 years.
“Thousands of our personnel stand shoulder-to-shoulder with Nato allies around the world. Those here in Afghanistan are just one example of Global Britain in action.”
The additional British Army personnel will help increase the existing contribution of the country to the UK-led Kabul Security Force (KSF).
The KSF has been deployed to carry out a major role in enabling Nato advisers in the city to undertake engagement with the Government of Afghanistan.
In addition, the Welsh Guards will continue to train the Afghan National Defence and Security Forces who are responsible for ensuring safety and security in the capital city of Kabul.