The UK is to devote more troops to UN peacekeeping, with personnel being sent to South Sudan, the BBC has learned.
The prime minister will detail the move at a summit in New York this month.
The extra troops will be deployed to the world's newest country, where thousands have been killed and millions displaced amid fighting between government troops and rebels.
The mission will be "significant" and involve boots on the ground, the BBC's UN correspondent Nick Bryant said.
Britain has long been a large financial contributor to UN peacekeeping missions and is the fifth highest provider of funds.
But its troop commitment has been relatively small - focused mainly on 250 soldiers based in Cyprus who wear the distinctive blue berets of the UN.
The size of the British contingent in South Sudan is not yet known. But it is expected to go beyond offering logistical support and an airlift capability, and will be a much tougher challenge, the BBC's UN correspondent added.
Both sides in the South Sudan conflict signed a peace deal, which it was hoped would bring to an end 20 months of conflict, last month. But fighting has continued.
Conflict between forces loyal to President Salva Kiir and his former deputy Riek Machar has forced more than 2.2 million people from their homes in the state, which broke away from Sudan in 2011.
At least seven ceasefires have been agreed and then broken.
The US administration has been saying for months that one way for European Union nations to address longstanding criticisms from Washington about inadequate defence spending would be to commit to more UN missions.