His fearsome reputation is summed up by his many nicknames: Darth Vader, Rasputin and The Monster.
As Jean-Claude Juncker’s chief of staff, Martin Selmayr is regarded as the power behind the throne in the Brexit negotiations to come, and the bad news for Theresa May is that he is an EU zealot determined to “punish” Britain for leaving the bloc.
Mr Selmayr, the man widely accused of leaking a damaging account of Mrs May to a German newspaper, is also close to senior figures in Angela Merkel’s party, raising questions about Germany’s role in briefing against Mrs May.
Tellingly, Mrs Merkel made a late addition to a speech she gave the day after Mr Juncker met Mrs May in Downing Street, to include a pointed reference to people in Britain having “illusions” about the Brexit deal.
The man who had told Mrs Merkel on the phone that Mrs May was “deluded” was European Commission President Mr Juncker, and critics of the former Luxembourg prime minister believe that his strings are being pulled by Mr Selmayr.
Ministers are particularly concerned about his links to the higher echelons of the German government. He is a member of Mrs Merkel's Christian Democratic Union party, and is friends with Peter Altmaier, her chief of staff.
Mr Selmayr, 46, is not a personal friend of Mrs Merkel - she is understood to have opposed his appointment - but Iain Duncan Smith, the former Conservative leader, believes he is trying to manipulate the German Chancellor into taking a tough line on Brexit.
He said: "They [Mr Selmayr and Mr Juncker] want to box Mrs Merkel in to a hard-line. They don't care if negotiations succeed, they are obsessed with the idea of the European Union - it's where their money, jobs and power come from."
A Whitehall source who has had frequent dealings with Mr Selmayr told The Telegraph: "He is incredibly clever, he is unbelievably hard-working, he is a consummate operator.
"He is also a true believer in the European Project and has taken Brexit very personally.
"He has always felt the UK was getting in the way of greater European integration and is clear that if you choose to leave there is a cost to doing so. His mindset is that of a lawyer, whose worldview is about rules and not political judgement."
British officials believe that Mr Selmayr, an arch-federalist, is determined to poison the negotiations in a bid to "punish" the UK for leaving the European Union.
While the EU referendum vote threw much of the European Union into despair Mr Selmayr reportedly declared at the time it was good news - because Eurosceptic Britain would be cut away - and would "help Europe to finally forge a new identity".
Last year he said that his "horror scenario" for 2017 would be Boris Johnson as Prime Minister, Donald Trump as US President and Marine Le Pen in France.
He has subsequently been credited with some of the European Union's most hardline negotiating positions, including its demand that Britain must pay a £50 billion Brexit divorce bill.
When the details of Mrs May’s meeting were leaked at the weekend, Mr Juncker was portrayed as the strong man in the face Mrs May's Brexit demands, saying that he told her on leaving Downing Street that he was "10 times more sceptical" about the success of the Brexit talks than before the meeting.
One minister told The Telegraph: "How was this leaked out to a German newspaper? I'm damned sure it was Selmayr. I think his reputation is one of a hard man and he's trying to prove it. It's a very stupid game to play."
Mr Selmayr says that his passionate commitment to the EU stems from a trip as a schoolboy to the First World War trenches in Verdun.
His paternal grandfather, Josef, served as a Lieutenant Colonel on Hitler's General Staff in the Balkans and and was later served four years for war crimes. He became one of the founders of the West German military secret service.
Mr Selmayr has spent much of his career surrounded by the bureaucracy of Brussels. He began his career as a lobbyist before working under Elmar Brok, an influential German MEP, then in 2004 became a spokesman for Viviane Reding, a former commissioner.
He subsequently developed a reputation as a formidable operator with a relentless work ethic, pushing aside Brussels veterans as he promoted his vision of a "United States of Europe".
In 2014 he ran Mr Juncker's campaign for the presidency of the European Commission. At the time Mr Juncker was considered an outsider, opposed by Britain and Germany.
Mr Selmayr succeeded in persuading Mrs Merkel to drop her opposition, leaving David Cameron isolated. He has risen to become the most high-profile chief of staff in the history of Brussels, feared by many of the 27 European Commissioners. He told the Financial Times: "Juncker is the good guy and I'm the bad guy. That's how it is."
Asked about his "monster" nickname, which even Mr Juncker has taken to using, Mr Selmayr said: "If you look into the history of Rasputin, that can be both flattering and not — Lenin can be flattering or not.
"If it means there is an efficient manager, somebody who is not a wimp, I’m OK with that. You can’t run the European Commission like a Montessori school.”