Merkel is due to make the speech to parliament Thursday, Steffen Seibert said, describing the rare move for a foreign leader as an "honour".
"She will hold a speech in front of both houses of the British parliament, in the Royal Gallery of the Palace of Westminster," he told reporters.
Seibert declined to detail the scope of Merkel's speech but described Berlin and London as important partners "not only bilaterally but also in Europe. That will naturally be a topic."
Cameron has pledged to renegotiate Britain's relationship with the EU and then hold an in-out referendum in 2017, provided he is re-elected next year.
More From This Section
After addressing lower House of Commons and upper House of Lords Thursday, Merkel will travel to Downing Street for lunch with Cameron followed by a joint press conference, and will then be received by Queen Elizabeth II at Buckingham Palace, Seibert added.
US President Barack Obama and former French president Nicolas Sarkozy are among the foreign leaders to have addressed members of both houses of the British parliament.