Britain's outgoing ambassador to the European Union has strongly criticized the government's approach to Brexit negotiations in a blistering farewell email to his staff.

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. The negotiations are expected to take two years.

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.

In probably the most damning passage, he 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.

The email was sent Tuesday, when Rogers resigned months earlier than expected. The government will need to name a new EU ambassador before talks get underway.

May, who took over after Prime Minister David Cameron resigned following his failure to convince Britons to remain in the EU in the June referendum, has said 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.

Her government also faces a lawsuit that would require it to obtain parliamentary approval before starting the process. The Supreme Court is expected to decide shortly whether to overturn a lower court's ruling that Parliament must approve before Article 50 is invoked.

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.