Up to 400 UK troops will deploy to South Sudan as part of a UN peacekeeping mission.
Defence Secretary Michael Fallon announced that up to 100 more service personnel would be sent to the country, following David Cameron's promise of 300 troops for the UN mission last year.
The personnel will help maintain the fragile peace in the African nation, with the additional deployment enabling the provision of a field hospital to support the peacekeepers from the UK and other countries.
The announcement comes as Mr Fallon hosts counterparts from around the world, including US Defence Secretary Ash Carter, for a peacekeeping conference in London.
Mr Fallon said: "This large-scale deployment underlines how we are stepping up our global commitments.
"Backed by a rising defence budget, it's part of our effort to tackle the instability that leads to mass migration and terrorism.
"It will help keep Britain safe while improving lives abroad."
The UK's work in the country has involved engineering projects and support to the UN Mission in South Sudan (UNMISS) headquarters.
The first UK personnel arrived in South Sudan in June, and the main body deployment is expected in the country next year.
As well as the UNMISS deployment, around 40 troops are already supporting a UN mission in Somalia, which is tackling the extremist group al Shabaab.
The number of troops deployed can rise up to 70 to accommodate short-term training teams.
The Ministry of Defence said the commitments equate to a doubling of support to UN operations.