Sir Ivan Rogers’ resignation as Britain’s EU ambassador, accompanied by a parting shot at the “muddled thinking” of his former bosses, has prompted demands that he be replaced by an official with a more positive view of Brexit.

But Sir Ivan’s supporters, including senior former civil servants, claim Theresa May is about to embark on Brexit without seasoned EU negotiators and surrounded by aides who do not want to hear bad news.

The ambassador’s colleagues say he grew “frustrated” that his warnings on the complexity of Brexit were being ignored. In a resignation email he urged diplomats in Brussels to continue to deliver messages that are “disagreeable to those who need to hear them”.

Sir Ivan’s abrupt departure and percussive resignation letter infuriated Number 10, with government officials telling the Daily Telegraph that he “jumped before he was pushed”.

Officials claimed that Boris Johnson, foreign secretary, wanted to replace Sir Ivan with someone who backed the Brexit cause “wholeheartedly”, while another said he or she should have “the same attitude [to Brexit] as the current government”.

Jacob Rees-Mogg, a Eurosceptic Conservative MP, said: “I think it is crucial that whoever represents us in Brussels is wholly committed to Brexit. Sadly, the impartiality of the civil service came into question during the referendum campaign, which made the position of the highly intelligent Sir Ivan difficult.”

John Redwood, the Tory backbencher, told Sky News Sir Ivan was “certainly associated with an unsuccessful renegotiation of our relationshiop with the rest of the EU. So I think we need a fresh person to help the prime minister and her senior ministerial team.”

Sir Ivan’s resignation surprised Mrs May, who will set out more details of her Brexit strategy within days. Sir Ivan said in his letter that even he did not know “what the government will set as negotiating objectives”.

Former civil servants expressed dismay at the resignation. Sir Simon Fraser, permanent secretary at the Foreign Office until 2015, said Sir Ivan’s departure would deprive the government of a key source of intelligence:

It’s very important for this country to have that sort of knowledge and understanding to achieve its objectives.

You don’t appoint [ambassadors] for what they believe. You appoint them for what they know. And I think what we need in Brussels is someone with experience who is going to be a real professional negotiator who will be sitting in the room with lots of other very experienced and knowledgeable negotiators and who will be able to hold his or her own in that negotiation.

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