The UK's ambassador to the EU urged British colleagues in Brussels to challenge "muddled thinking and... speak truth to power" as he quit ahead of Brexit talks, it has emerged.
Writing to staff, Sir Ivan Rogers said ministers needed to hear "unvarnished" and "uncomfortable" views from Europe.
Earlier it emerged Sir Ivan would be leaving his post several months early.
The government said he had quit so a successor could be in place before Brexit negotiations started.
Sir Ivan's note to staff, obtained by the BBC, confirmed this but also warned the "government will only achieve the best for the country if it harnesses the best experience we have".
Downing Street said it would not comment on the resignation email, although Sir Ivan was "free to express his own opinions".
But former Conservative leader Iain Duncan Smith, who campaigned for Leave during the referendum, told BBC Radio 4's Today programme that when a civil servant "starts going public", ministers "can no longer trust that individual".
Guy Verhofstadt, the European Parliament's chief Brexit negotiator, tweeted: "Best Wishes to Sir Ivan Rogers, a much-respected UK civil servant in Brussels - who knew what he was talking about."
This article was originally published by the BBC - http://www.bbc.co.uk/news/uk-politics-38503020