French President Jacques Chirac will represent Germany on the last day of this week's European Union summit, according to German officials in Berlin.
The two men have developed a close working relationship
Mr Chirac has been asked to stand in for Chancellor Gerhard Schroeder who will fly back to Berlin for a crucial parliamentary vote instead.
"This is unprecedented and underlines how close we are," a German official told the AFP news agency.
The BBC's Berlin correspondent says the decision shows the Franco-German motor is better oiled than ever.
Both countries opposed the US-led invasion of Iraq, and the fact that Germany is content for France to speak on its behalf underlines how close their positions now are on crucial international issues.
Mr Schroeder and his Foreign Minister, Joschka Fischer, will fly back from Brussels for a key parliamentary vote on their centre-left government's labour reform measures.
Mr Schroeder wants to be on the spot after a threatened rebellion within his own party.
The two will leave the EU summit late on Thursday, leaving President Chirac to deal with any German business in the final session before the meeting finishes mid-way through Friday morning.
A recent summit greeting showed the warmth of Franco-German ties
"If Germany should have any input to make for Friday's conclusions, Chirac will present them on behalf of Germany," said a German official.
The French and German leaders cemented their personal alliance with their opposition to the war in Iraq and have closely co-ordinated their positions on the EU constitution.
They have also put forward a common front in defending their budget deficits which exceed EU eurozone limits.
The BBC's William Horsley says the breathtaking decision that Mr Chirac will speak for Germany can be seen as a logical consequence of the two governments' pledge in January this year to move in time towards a complete union or merger of the two countries.
It raises a host of practical questions, he says, but above all it signals to other heads of government that France and Germany are ready to act radically to build a politically united Europe.