British Prime Minister Theresa May outside Number 10 Downing Street on May 10, 2017 | Justin Tallis/AFP via Getty Images Theresa May appoints ex-EU official as top aide Peter Hill will join the British prime minister’s team in Downing Street.

LONDON — Theresa May has appointed a former European Commission official and trade expert who worked under Peter Mandelson in Brussels as her most senior civil servant inside Number 10.

Peter Hill, who worked in Mandelson’s cabinet when he was European trade commissioner, will become May’s principal private secretary in Downing Street, having served as director of strategy at the Foreign Office since 2013, the prime minister’s official spokesman told POLITICO.

The principal private secretary runs the prime minister’s private office in Downing Street, “a hugely important job” in the government machine, according to one senior government official, who spoke to POLITICO on condition of anonymity.

Hill replaces Simon Case, who was appointed to to the U.K.’s Permanent Representation in Brussels (UKREP) in March. Case is now director general for the U.K.-EU partnership, coordinating the Article 50 process and negotiations for a future agreement, working under Britain’s ambassador to the EU Tim Barrow. Hill is expected to officially commence in his new role shortly, the senior official said

Hill’s appointment adds a wealth of Brussels knowledge to May’s top team. He is one of the few people in the top ranks of British government with experience of trade negotiations, and his appointment will reassure critics of the government’s hard Brexit stance, who have been alarmed at the increasingly belligerent anti-Brussels rhetoric emanating from No.10 in recent days.

Peter Hill replaces Simon Case, who was appointed to to the U.K.’s Permanent Representation in Brussels (UKREP) in March.

Last week May railed against “the bureaucrats of Brussels,” accusing European officials of attempting to influence the U.K.’s general election.

However, the senior government official cautioned not to “read the runes,” adding that he was appointed simply as the best candidate in a highly competitive Whitehall recruitment process.

Appointing a former European trade official signals the paramount importance to May of securing a free trade agreement with the European Union and will be welcomed by those in her government, including Chancellor Philip Hammond, who are not content to see the U.K. leave the EU with “no deal,” as May as previously threatened it might.

Hill is a known commodity in Brussels, having worked at the European Commission's department for trade between 2006 and 2009 under Mandelson and his successor Catherine Ashton.

Mandelson, a Labour peer who campaigned prominently for Remain and an outspoken critic of the government’s Brexit strategy, warmly welcomed the appointment.

“Peter is an excellent choice for the prime minister,” he said. “He is clever, quick, knowledgeable and collegiate, and thoroughly loyal to whoever he works for.”

The move has not just pleased Remainers. Former UKIP MP Douglas Carswell hailed it as a sign that trade is becoming a core priority of the British government.

“It shows that right at the heart of power, Theresa May takes the business of trade seriously,” he said. “With this guy Hill there, I think it shows that there is this great pivot going on in the official mind toward something more ambitious than there has ever been before.”

Hill confirmed his departure from the Foreign Office in an email to staff two weeks ago.

Whitehall sources described him as a highly-regarded and effective operator. He has experience working under May in the Home Office’s security and counterterrorism team, before joining the Foreign Office in 2013.

“Theresa’s first instinct is to look to people she knows already,” said one former high-ranking Whitehall official. The appointment was a coup for the Foreign Office, the ex-official added, pointing out that the role of principal private secretary is typically taken by an official with a Treasury background.

The role has a distinguished pedigree, with previous holders including Jeremy Heywood, Britain's chief civil servant, and Oliver Robbins, who had the job under Tony Blair and now leads David Davis’ Department for Exiting the European Union, and will act as May’s sherpa in the Brexit negotiations.

“Hill seems to be getting good vibes in Whitehall,” a former high-ranking official said.

Crispin Blunt MP, chair of the House of Commons foreign affairs committee, said: “The times I’ve met him he’s been immensely impressive, doing thought leadership for the Foreign Office. Extremely bright and capable, I think he’s a very good choice.”

This story has been updated to add a comment from Douglas Carswell.