Fischer, who retired from parliament in June to take up a teaching post at Princeton University, is expected to represent Germany in an unofficial capacity, the Sueddeutsche Zeitung will report on Saturday, according to an advance extract.

Fischer, whose current wife is Iranian, was invited on the four-day trip by Hassan Rohani, Iran's former chief negotiator on its nuclear programme.

During his term of office as foreign minister from 1998 to 2005, Fischer, a Green, was the most popular politician in Germany.