Theresa May will give a major speech on the Government's plans for Brexit amid accusations of lacking a proper exit strategy.
Article 50 is due to be triggered in March, and ahead of the start of negotiations, Sir Ivan Rogers resigned on Tuesday, claiming there was "ill-founded arguments and muddled thinking" over what sort of deal the UK wanted.
Mrs May has attempted to curtail the row created by his surprise resignation by agreeing to a replacement just over a day after Sir Ivan gave in his notice.
Sir Tim Barrow, a former ambassador to Russia, will serve as the ambassador to the EU - a man described by No 10 as a "seasoned and tough negotiator" whose expertise will help to make Brexit a success.
A Downing Street spokesman said: "We are delighted Tim Barrow is taking up this role. A seasoned and tough negotiator, with extensive experience of securing UK objectives in Brussels, he will bring his trademark energy and creativity to this job - working alongside other senior officials and ministers to make a success of Brexit."
Foreign Secretary Boris Johnson has backed him, saying Sir Tim was "just the man" to secure the best deal.
He said: "Tim Barrow has been invaluable since I joined the Foreign Office in July and I want to personally thank him for his relentless energy, wise counsel and steadfast commitment.
"He is just the man to get the best deal for the UK and will lead with the same skill and leadership he has shown throughout his career."
Theresa May's speech - which could address all of these factors - is expected later this month, with both Boris Johnson and Brexit Secretary David Davis to contribute to it.