By Elizabeth Piper and Guy Faulconbridge
LONDON (Reuters) - British Prime Minister Theresa May appointed a senior career diplomat as envoy to the European Union on Wednesday to replace an ambassador who quit with a scathing resignation letter that exposed frustration among officials over her strategy.
Tim Barrow, political director at the Foreign Office, will take up the post next week following the abrupt departure of Ivan Rogers, who told staff in his resignation letter they should "speak the truth to those in power".
The implicit criticism of the government's approach in Rogers' letter put rare strain on rules that shield the politically neutral civil service from elected leaders.
The selection of Barrow, a 30 year veteran diplomat, could disappoint some Brexit campaigners who would like to see a known eurosceptic in the post. But it could help reassure Britain's cadre of civil servants that their expertise is still valued.
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Barrow, a former ambassador to Moscow who served earlier in his career as first secretary at Britain's embassy in Brussels, is not known to have taken a strong public position on Brexit.
In a statement released by May's Downing Street office, he said he looked forward to joining the new government department tasked with overseeing the exit from the European Union, "to ensure we get the right outcome for the United Kingdom as we leave the EU".
Downing Street described him as "a seasoned and tough negotiator, with extensive experience of securing UK objectives in Brussels".
May intends to launch the two-year process of negotiating to leave the bloc by the end of March, beginning what is expected to be some of the most complicated international talks Britain has engaged in since World War Two. She has so far said little publicly about Britain's negotiating position, arguing that to do so would weaken London's hand in talks.
Her political opponents say the government underestimates the task and has failed to take into account the position of European leaders, who say they will not give Britain access to the EU free trade zone if it closes its borders to EU citizens.
In his undiplomatically worded resignation letter, Rogers said May's negotiating objectives were as yet unknown. He told his staff: "I hope you will continue to challenge ill-founded arguments and muddled thinking".
May's opponents said his departure would deprive Britain of crucial expertise about Europe at the time when it was needed most. But Brexit supporters described his comments as sour grapes and said he should be replaced by someone who was more positive about Britain's prospects outside the EU.
With May's end of March deadline to trigger the formal EU divorce procedure approaching, her reticence over her strategy may be contributing to a lack of coordination among staff in government departments, say experts on the British bureaucracy.
Robyn Munro, senior researcher for the Institute for Government which is close to the civil service, said there were questions over what kind of structure Britain would put in place to manage the negotiations.
The future of Rogers looked precarious late last year when a report was leaked that he had told ministers that a post-Brexit trade deal with the EU could take 10 years to finalize. Prominent eurosceptics in May's Conservative Party accused him of being overly "pessimistic" before the talks.
One source in parliament said Rogers had been victim of a breakdown in relations not just with ministers, but within the civil service after the new Department for Exiting the EU took precedence over his Brussels-based mission.
Other diplomats said Rogers, who had been central to a difficult renegotiation of the terms of Britain's EU membership led by former prime minister David Cameron before last year's referendum, was frustrated that his message that the talks would be tough was not getting through.
But pro-Brexit lawmakers said it was right that officials who lacked enthusiasm for quitting the EU should step aside.
"Some have been passionately committed to the previous pro-EU consensus and they are in real grief, like many others we know. If they cannot adapt to the new policy, then they are right to go," said Conservative lawmaker Bernard Jenkin, who campaigned for Britain to leave the European Union.
(additional reporting by Estelle Shirbon and Alastair Macdonald in Brussels,; editing by Peter Graff)