David Cameron has astonished Berlin by looking to make his controversial Europe speech on the same day that France and Germany stage lavish celebrations marking their postwar reconciliation.
To add to the diplomatic drama, Mr Cameron is considering making his speech in Germany on the day hundreds of French and German politicians gather in Berlin to mark the 50th anniversary of the Elysee treaty.
The office of Chancellor Angela Merkel is understood to be in direct contact with Downing Street about the preferred date. One senior German official said the timing was "quite extraordinary".
"It is insane," said Ulrike Gurot at the European Council on Foreign Relations. "Mr Cameron should have some decent respect. It is a sulky reaction, and an attempt to steal the headlines."
"Choosing the 50th anniversary of the most important treaty between France and Germany to make a big anti-EU speech is a grotesque perversion," added Denis MacShane, a former Labour Europe minister. "It's like a religious day for them."
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Mr Cameron is expected to decide this weekend precisely when and where he will set out his vision of a looser British relationship with the EU, which he would then put to a referendum.
Downing St favors January 22 because it falls between US President Barack Obama's inauguration on January 21 and the Davos World Economic Forum starting on January 23.
British officials insist the provisional date "is not intended" to be provocative. They argue that Mr Cameron will make the case for Britain staying in the EU – albeit on new terms – and will stress the project's contribution to postwar peace.
The choice of venue is also still open. Mr Cameron is considering making the speech in the Netherlands, an ally on free-trade issues, or Brussels.
British prime ministers have a long tradition of making big European speeches on the continent, from Winston Churchill in Zurich in 1946, to Margaret Thatcher in Bruges in 1988 and Tony Blair in Warsaw in 2000.
Meanwhile Vince Cable, Lib Dem business secretary, welcomed as "very helpful as well as very timely" a warning from the US this week that Britain should not put its EU membership at risk. He added that jeopardising Britain's EU role was "a massive disruption and deeply unhelpful" – a view echoed on Friday by Lord Heseltine, the former Tory deputy prime minister, in an FT interview .