A civil servant previously tasked with solving the Irish border issue in Brexit talks has been given a leading role in the Government's response to Covid-19.
Simon Case, who is on a secondment to Whitehall, has been made permanent secretary at 10 Downing Street.
Mr Case also served as the Duke of Cambridge's private secretary.
The Prime Minister's official spokesman said: "Simon is working on the coronavirus response in No 10 as a permanent secretary. Simon is with us temporarily from the Royal household."
"He is specifically co-ordinating the coronavirus response. Coronavirus is obviously a very significant piece of cross-Government work."
Mr Case will report to Cabinet Secretary Sir Mark Sedwill, the spokesman said.
A former career civil servant, Mr Case had served as principal private secretary to David Cameron when he was Prime Minister and his successor Theresa May during 2015-17.
Before he began working for William, in July 2018, he was Director General Northern Ireland and Ireland in the Department for Exiting the EU.
Mr Case led a team of key officials working on what is arguably the most challenging issue facing the department - the Irish border question.
Separately, Northern Ireland Secretary Brandon Lewis said the Government has followed "the best advice that is out there" in its handling of the Covid-19 outbreak.
Responding to criticism from Francis Crick Institute chief executive Sir Paul Nurse that it was unclear who was in charge, Mr Lewis told BBC Radio 4's Today Programme: "I just wouldn't agree with that.
"I think what we have seen through this actually is we as a Government have been very clear with people, very transparent with people.