Sir Ivan Rogers, the UK's Permanent Representative to the EU, has resigned.
Here's a guide to who he is, his responsibilities and how significant his decision to quit could be.
What was his job?
In his role, Sir Ivan had overall responsibility for the UK's team which represents the nation in negotiations that take place in Brussels.
He represented the UK at the weekly meeting of the Committee of Permanent Representatives, also called Coreper, which prepares the work of the Council of the European Union.
Put simply, he was the UK's top diplomat in Brussels.
Was his resignation expected?
No, his decision to quit has come out of the blue but his private warnings about the length of time it could take for the UK to strike a new trade deal with the EU are thought to have placed a strain between Sir Ivan and some members of the Government.
How significant is his resignation?
Potentially very. The timing of Sir Ivan's decision to quit could be critical because the Government is due to trigger the Article 50 process for the UK's divorce from the EU before the end of March.
That means Prime Minister Theresa May will likely need to find a replacement to take on the job before the Government embarks on what many people believe will be the most complex set of negotiations faced by the UK for decades.
How did he get the job?
He was appointed to the role by David Cameron in November 2013.
What did he do before his appointment to the role?
Sir Ivan previously held a number of roles within the civil service including principal private secretary to the prime minister, director of budget and tax policy at the Treasury and chef de cabinet to the vice president of the European Commission.
He also worked as head of UK public sector business at Barclays Capital. He was awarded a knighthood in 2016 for services to British, European and International Policy.