Downing Street has called the UK's new ambassador to the European Union a "seasoned and tough negotiator", who will bring "energy" to Brexit talks.
Sir Tim Barrow takes on the role with discussions with the EU expected to start soon and promised to work for the "right outcome".
He replaces Sir Ivan Rogers, who quit earlier this week, accusing ministers of "muddled thinking".
Some MPs have accused Sir Ivan of being "half-hearted" towards Brexit.
Meanwhile, Prime Minister Theresa May is expected to give what is being billed as a "major" speech on Brexit within the next few weeks.
Sir Tim, UK ambassador in Moscow from 2011 to 2015 and an adviser to several previous foreign secretaries, said he was "honoured" to be appointed permanent representative to the EU and promised to work for "the right outcome" for the UK after Brexit.
Downing Street said he had "extensive experience of securing UK objectives in Brussels" and would "bring his trademark energy and creativity to this job".
The resignation of Sir Ivan Rogers has revealed more than the difficulty and complexity of Britain's EU divorce.
It has highlighted wider strains in Whitehall between some mandarins and some ministers, up to and including Theresa May.
Mandarins and ambassadors perennially advise more junior mandarins on the importance of speaking truth to power.
On this occasion, Sir Ivan's leaked farewell memo can fairly be read as a protest and a warning. Concern is growing among some high-ranking officials that ministers don't understand or won't admit the scale of the task they're facing.
Foreign Secretary Boris Johnson and Brexit Secretary David Davis both welcomed Sir Tim's appointment and Tom Fletcher, a former UK ambassador to Lebanon, told BBC Radio 4's Today programme: "It's the toughest negotiation in our lifetimes and I think he is up to it. I have seen him in Brussels. He knows the corridors, he knows the characters.
"But actually more importantly I saw him in Moscow where he was incredibly resilient as ambassador there, dealing with (Vladimir) Putin in a very testing time in our relationship and Tim had a reputation of being bulletproof out there."
Former UKIP leader Nigel Farage was less enthusiastic, tweeting: "Good to see that the government have replaced a knighted career diplomat with... a knighted career diplomat."
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