Britain claims that Lord Elgin had permission from the ruling Turkish authorities to take them.
The British also contend that the marbles have received better treatment at the British Museum, where they are safe from the polluted Athens air, which has damaged other Greek treasures.
Voulgarakis remarked that a space in the new Acropolis museum, which is due to be completed next year, has been reserved for the Parthenon sculptures.
"The demand for unification of the Parthenon sculptures originates exclusively by moral reasons and not by a nationalistic obsession," he said.
The University of Heidelberg stressed in a statement that the transfer of the fragment is "a special case that should not be used as a precedence for other monuments and works of art."
But the German gesture is likely to cause controversy among museums worldwide.
In 2002, when negotiations were underway to return a Parthenon fragment held at the Salinas Regional Archaeological Museum in Palermo, Sicily, directors of several major museums around the world deplored the move, saying in a statement that it could destabilize the entire museum system.
