Angela Merkel will take tea with Britain's Queen Elizabeth and address parliament today as David Cameron tries to win the German chancellor's backing for EU reform.
Cameron is rolling out what the British press called the "reddest of red carpets" for Merkel, in contrast to a low-key welcome for French President Francois Hollande last month.
In the leader of Europe's top economy, Cameron sees a potential ally for his plans to change the EU's treaties before holding a referendum on Britain's membership in 2017.
Merkel's speech to members of both chambers of parliament is the first by a German leader since president Richard von Weizsaecker did so in 1986.
US President Barack Obama and former French president Nicolas Sarkozy are among the other foreign leaders to have spoken to the Houses of Commons and Lords.
The German chancellor will also hold talks and a press conference with Cameron, before going to Buckingham Palace to meet the queen.
The Guardian reported yesterday that Merkel may be prepared to back some concessions in order to keep Britain in the EU.
Citing a senior Berlin source, the paper said that Merkel may consider backing Cameron's call for assurances that euro zone members will not gang up against non-members when voting on the future of the single market. It also said she may be open to limited opt-outs for Britain and a less rigid implementation of EU rules, but analysts warned that Merkel was restricted in what she could offer her fellow conservative.
"I think her hands are very, very tied by her 'grand coalition' in Berlin," said Simon Hix, professor of European politics at the London School of Economics.
Merkel's conservative Christian Democrats (CDU) leads a shaky two-month-old coalition with their Bavarian CSU sister party and the centre-left Social Democratic Party (SPD).
Germany has been lukewarm towards Cameron's plans to change the 28-member bloc's treaties, which are opposed by France and other EU nations.
Merkel's spokesman Steffen Seibert said last week that Germany wants an "active, strong Britain in the EU" and added that it would "be a topic" of her speech.
Cameron will be hoping to capitalise on his strong personal relationship with Merkel but a recent improvement in the relationship between Merkel and Hollande could weaken his bargaining position.
Germany's priority, and the rest of Europe's, remains fixing the euro zone.