A decision on a new US ambassador is likely to be taken within days, with David Miliband, Peter Mandelson and Cathy Ashton all on the shortlist, the Guardian understands. Whitehall sources said Downing Street is determined to press ahead with the appointment of a new US ambassador now that Donald Trump has been confirmed as the next US president. The sources said the three figures had been on the shortlist for months, and that a final choice of who to ask would be made imminently. A senior government source said one school of thought in Whitehall was that a former politician might be the best option given how much Trump is opposed to bureaucrats. The new US ambassador will replace Karen Pierce, who has been in the job since 2020. Pierce has had her term extended to January to help cover the transition from the Biden administration to Trump. Pierce is understood to be keen to stay in post for as long as possible. Mandelson, an architect of New Labour, a former business secretary and a former European Union trade commissioner, co-founded an advisory firm after leaving public office and is running to be chancellor of Oxford University. Miliband, the brother of the energy secretary, Ed Miliband, is a former Labour foreign secretary and leadership contender who runs the International Rescue Committee charity in the US. Ashton is a Labour peer and former minister who also served as a European Union trade commissioner. In 2009, she became the EU’s foreign policy chief, subsequently winning praise for her role in relations with Kosovo and Iran. The role of US ambassador will be a crucial post as the UK deals with a Trump administration that has threatened to bring in protectionist trade policies. Rishi Sunak had appointed Tim Barrow, formerly the national security adviser, as the US ambassador but this was cancelled by Starmer after taking office, with sources saying he wanted his own candidate in place. On Pierce’s transition period, the chancellor of the duchy of Lancaster, Pat McFadden, told Sky News on Thursday: “At the moment, she is doing an excellent job. She’s got the full confidence of the British government, and we want her to keep on doing the job that she’s doing. And I think she’s going to be a very important interlocutor and adviser for the UK government in this period of transition. “I can’t say exactly when her term will end. She’s been doing it for a few years, but she’s there for the moment, she’ll be there for a while, and she’s doing an absolutely fantastic job for our country.” Pierce posted to X on Wednesday after Trump was announced as the US election winner: “We look forward to deepening our already profound and successful partnership as we deal with the challenges of the 21st century.” She was appointed as UK envoy to the UN in New York in 2018, and then in 2020 she was appointed ambassador to Washington.