The UK has named Dame Barbara Woodward as its permanent representative to the United Nations, its most senior diplomat to the New York-based body.
Dame Barbara, currently British ambassador to China, is a previous international director of UK Border Force.
She replaces Dame Karen Pierce, who was named ambassador to the US in February.
The UK is a founding member of the UN, and one of five permanent members of its security council.
Announcing the appointment, Foreign Secretary Dominic Raab said Dame Barbara would bring "formidable intellect and dynamic diplomatic skills" to the role.
He added she would help "deliver for Britain, and forge the cooperation we need with our international partners to tackle the toughest global challenges we face".
Dame Barbara, who has worked in several Foreign Office roles since first joining in 1994, will be the second woman in the role for the UK.
The first was Dame Karen, who then went on to become the first female British ambassador to Washington.
The BBC's James Landale said it now meant the UK had two women in key positions at the top of its diplomatic service.
Dame Barbara Woodward said she was "delighted" to take up the role at a time of "pressing global challenges" for the international system.
"The UK has a vital role to play as a permanent member of the UN Security Council and the UN's third largest donor," she added.
The UN is an international organisation formed at the end of World War Two in 1945, with the aim of keeping peace around the world.
It now includes 193 countries that are full members and two non-member states - the Holy See (the area under the Pope's jurisdiction) and the State of Palestine.
The UK - alongside China, France, Russia and the US - is one of five permanent members of the security council, with the power to veto resolutions.
The council also has 10 non-permanent members, elected for two years each.