Theresa May is preparing to make a major speech on Brexit in which she will reportedly say Britain will pull out of the single market if the European Union fails to make concessions on freedom of movement.
The announcement comes amid accusations by the disgruntled outgoing top UK civil servant in Brussels that the government lacks an exit strategy.
Foreign Secretary Boris Johnson and Brexit Secretary David Davis are contributing to the content of the address, which is expected later in January.
May moved swiftly to draw a line under the row sparked by the surprise resignation of UK permanent representative Sir Ivan Rogers by agreeing his replacement within 36 hours of his notice to quit.
He is being succeeded next week by career diplomat Sir Tim Barrow, a former ambassador to Russia described by No 10 as a “seasoned and tough negotiator” who will help the Government make a success of Brexit.
The appointment was welcomed by Labour but Ukip said the role should have been given to a “committed Brexiteer”.
According to The Daily Telegraph, the prime minister will use her speech to signal she is not afraid of a so-called hard Brexit that would take the UK out of the single market.
The move would delight the most eurosceptic Brexit campaigners and worry those who backed Remain in equal measure.
In a fiery resignation letter, Sir Ivan had hit out at the “ill-founded arguments and muddled thinking” of politicians and said civil servants still did not know the Government’s plans for Brexit.
It sparked a war of words, with prominent Brexiteers Iain Duncan Smith and Peter Lilley accusing the diplomat of “sour grapes” while former senior civil servant Lord Ricketts attacked the “denigration” of the long-serving mandarin.
Brussels officials said Sir Ivan’s resignation meant the UK had lost a “professional” diplomat who had always “loyally defended” his Government.
Boris Johnson said Sir Tim was “just the man” to secure the best deal for the UK.
“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 said.
“He is just the man to get the best deal for the UK and will lead UKRep with the same skill and leadership he has shown throughout his career. I wish him all the best.”
But former Foreign Office diplomat warned there was “a policy vacuum” at the heart of the Brexit plan. “It’s not surprising,” he told BBC Radio 4’s Today programme. “This is a gigantic enterprise that’s been taken on and needs a lot of thought. I think at the moment probably the atmosphere is difficult because people don’t know where they are going. You need to have a sense of direction.”
He has had two previous stints working as part of the UK’s Permanent Representation to the EU (UKRep).
Davis said: “His knowledge of Brussels means he will be able to hit the ground running at a vital time, and steer UKRep throughout the negotiation period. I am confident that with his help, the UK will be able to forge a new relationship with the EU that works to the mutual benefit of both sides.”
Sir Tim said he was “honoured” to be appointed to the crucial role.
A Downing Street spokesman said: “We are delighted that 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.”
Ukip former leader Nigel Farage said: “Good to see that the Government have replaced a knighted career diplomat with ... a knighted career diplomat.”
Labour called for the Government to set out a clear timetable for the publication of its Brexit plans in the wake of the resignation.
The party’s Brexit spokesman Sir Keir Starmer said Sir Ivan’s resignation was likely to be a “significant loss” for Britain and raised “a number of serious questions” about the Government’s preparations for the talks, which Mrs May has said she will trigger under Article 50 of the EU treaties before the end of March.
He added: “I welcome the appointment of Sir Tim Barrow as the UK’s new Permanent Representative to the EU. It is of course vital that there should be no vacuum in such an important role and that the new Permanent Representative should be someone with a strong and distinguished record of service as a diplomat.
“But a number of fundamental questions remain unanswered. In particular, Sir Ivan Rogers’ confirmation that the Government lack a plan for Brexit and his statement that the UK does not have a proper and effective negotiating team in place.”