February 25, 2017 0351 GMT
Ivan Rogers said Prime Minister Theresa May's government has been hindered by "ill-founded arguments" and "muddled thinking" in its approach to the upcoming talks to pull Britain out of the EU, AP reported.
His harsh critiques, likely to embarrass the government in the run-up to complex negotiations with the other 27 members of the EU bloc, were made public when the email was published by British media Wednesday.
Rogers' abrupt departure leaves Britain without an envoy to the EU bloc and reveals the deep uncertainties within the government about how to best protect Britain's national interest as it leaves the EU.
EU Commission spokeswoman Natasha Bertaud said Rogers would be missed.
"We regret the loss of a very professional, very knowledgeable, while not always easy interlocutor and diplomat who always loyally defended the interests of his government," she said.
In probably the most damning passage, Rogers said that senior civil servants don't know what the May government's priorities are in the talks, which will set the course for Britain's relations with Europe once it leaves the bloc.
"We do not yet know what the government will set as negotiating objectives for the UK's relationship with the EU after exit," he wrote.
That implies he is unsure whether May seeks to keep Britain in the single market or the customs union, or to have the country sever those economic relationships.
Rogers also said that Britain, unlike its EU adversaries, does not have the "serious multilateral negotiating experience" needed for the task.
The email was sent Tuesday, when Rogers resigned months earlier than expected.
May took over after Prime Minister David Cameron resigned following his failure to convince Britons to remain in the EU in the June referendum. She says Britain will formally trigger Article 50 — the legal process to kick off the process of leaving the EU — by the end of March.
She has repeatedly refused to spell out the government's goals, saying to do so would weaken its hand in negotiations.
Rogers had earlier warned it would take longer than expected to reach new trade deals with Europe and that the process might ultimately fail because it would require parliamentary approval from each member state.