For Barrow's 27 peers from the other member states, it was business as usual on Wednesday morning with their weekly meeting at the European Council. On the agenda of the body, known by its French acronym Coreper, was a scheme to provide emergency aid during natural disasters and EU legislative proposals to control new psychoactive substances, commonly known as legal highs.
For Barrow, however, this was a day like no other.
Probably never before has a British envoy to the European Union been greeted by quite so many snapping photographers and yelling camera crews when he stepped from the ambassadorial Jaguar on to the EU's red carpet.
Formerly "our man in Moscow", the 53-year-old diplomat, appointed to the Brussels Brexit hot seat in January after the dramatic resignation of his disgruntled predecessor, was saying nothing as he strode past the Union's blue and gold star flag.
Officials would not say whether the letter from the prime minister confirming Britain's intention to quit the bloc under Article 50 of the EU treaty was in his well-worn black briefcase. Nor would they reveal how the missive, signed by May in 10 Downing Street on Tuesday, had arrived in Brussels.
After his fellow Coreper ambassadors break for lunch, Barrow has an appointment at 1:20 pm (1120 GMT) with European Council President Donald Tusk, the EU summit chair and former Polish prime minister, to make the formal delivery of the letter.
That will set the clock ticking to Britain's departure, in principle on March 29, 2019, a Friday, although the precise date could be fine-tuned by negotiation. From now until then, Barrow will have to get used to Coreper frequently meeting without him as the EU27 takes on a legal character to exclude Britain.
Barrow's handover of the letter, which EU officials expect will offer a positive approach to negotiations and set out over several pages May's existing demands, is due to take place in Tusk's offices on the top floor of the Europa Building.
Opened just this year amid criticism of its cost and garish internal colour schemes, the building which senior EU officials jokingly refer to as "Tusk Tower", was planned in happier times, when the Union seemed set for ever more expansion, taking in a dozen new members, mainly in formerly Communist eastern Europe.
On Wednesday, it will see its first great historic occasion but hardly one its designers foresaw. Nine months after Britons shocked the bloc by voting to leave in a referendum, Barrow will launch the unprecedented shrinking of the European Union.
On the other side of Brussels' Rue de la Loi (Law Street), in The Old Hack, a British-style pub, members of the European Parliament from the UK Independence Party will be celebrating. The anti-EU campaigners are throwing a "Brexit Party".
(Editing by Mark Trevelyan)
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