The surprise nomination places an architect of Europe's post-crisis financial regulation at the helm of the technical talks for Britain to leave the EU. Jean-Claude Juncker, the commission president, said he wanted an "experienced politician for this difficult job".
The UK government's cool response to the decision reflected heightened concerns that the commission will be an awkward negotiating partner. An official statement did not mention the Frenchman by name and was drafted to suggest that the commission was the least important of three interlocutors in the Brexit talks.
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"We look forward to working with representatives from the member states, the council and the commission to ensure an orderly departure of the UK from the EU," a UK spokesman said.
Some City financiers were even more scathing about the appointment. One chairman said: "My initial reaction was 'Oh, God.' It's clearly provocative."
"I can't see how it could be worse," said another senior banking industry figure, who has worked in Brussels. "It's incredibly provocative. This is Juncker's revenge on Britain."
Mr Barnier has always insisted that he went to great lengths to avoid alienating Britain or the City when he was the EU's single market commissioner from 2009 to 2014.
But his interventionist instincts and Gaullist style at times made him the nemesis of the UK Treasury. Britain was outvoted for the first time on a big EU financial services law in 2013 when it objected to a revamp of bank capital rules.