The appointment means that Sir Kieran, a former British ambassador to Turkey, and High Commissioner in Kenya, will take a key position in the constellation of officials surrounding Mr Annan. He will act as head of the important political affairs department and will be a voice close to the Secretary General's ear on security matters.
The political affairs post was occupied by another Briton, Marrack Goulding. Mr Goulding announced before Christmas that he planned to leave the UN to become warden at St Antony's College, at Oxford University.
While each permanent member of the Security Council is traditionally guaranteed an important under-secretary slot, there had been nervousness in Whitehall that the departure of Mr Goulding could threaten Britain's hold on political affairs. This was deepened by concerns that France could be rewarded for agreeing at the last moment in December to withdraw its support for a second term for Mr Boutros Boutros Ghali and support Mr Annan.
The new Secretary General has shied from any radical shake-up of the UN's top tiers, leaving many of Mr Boutros Ghali's old team in their places.