Sir Ivan Rogers, Britain's ambassador, has unexpectedly resigned from his role, just months before negotiations with the EU were due to begin, TASS reported.
Rogers was due to finish his fixed-term role in October, but has told staff this afternoon that he will step down earlier than expected.
The news came as the government prepares to trigger Article 50 in March. Having moved to Brussels in 2013, Rogers is one of the UK's most experienced EU negotiators.