Sir Ivan Rogers has suggested the UK faces a disadvantage in its Brexit negotiations because British officials lack the required experience.
Rogers, the outgoing EU ambassador, also blasted the "ill-founded arguments and muddled thinking" around Brexit in his leaked resignation letter.
Rogers resigned yesterday in a surprise move, just weeks after warning that a post-Brexit EU trade deal could take up to a decade to finalise - and even then it could be rejected by other member states.
In the email, which The Times and the BBC have obtained, he revealed the reason for stepping down was to ensure that his replacement could be in position when Article 50 is triggered.
Rogers also urged his civil servants to "speak the truth to those in power" - perhaps implying that ministers don't always want to hear the harsh realities of Brexit.
Turning to the forthcoming negotiations, he said the UK was at a disadvantage because Brussels has "serious multilateral negotiating experience" whereas this is "in short supply in Whitehall."