Sir Ivan Rogers has resigned from the Civil Service - just days after stepping aside from his role as UK ambassador to the EU.
On Tuesday, Sir Ivan quit as Britain's top Brexit negotiator in Brussels, criticising politicians' "ill-founded arguments and muddled thinking."
He said the Government lacked an agreed EU exit strategy and revealed civil servants still do not know Theresa May's Brexit priorities.
Former ambassador to Moscow Sir Tim Barrow was elected to replace Sir Ivan on Wednesday.
The Foreign and Commonwealth Office confirmed Sir Ivan's resignation on Saturday.
A spokeswoman said: "Sir Ivan Rogers resigned as UK Permanent Representative to Brussels on 3 January."
The spokeswoman added: "He did not seek any further Civil Service appointment and has therefore resigned from the Civil Service with immediate effect.
"We are grateful for Sir Ivan’s work in Brussels and across a number of other senior positions in the Civil Service.”
Appointed by David Cameron in 2013, it was anticipated Sir Ivan would play a key role in Brexit talks, which are expected to start within months.