The event will be held on August 19 and will include an ecumenical service in the Lutheran church of Sopron, close to where 600 East Germans made it across the Austria border, leading to the fall of the Berlin Wall.

The Hungarian government says that both Orban and Merkel are expected to give speeches but the chancellor's office has not confirmed that she will be taking part.

Moves to take down the fence along the border, which had for over two decades divided the West from the Communist bloc,  begun in Hungary in May 1989.

On August 19, the border was to be symbolically opened for three hours but minutes before the gates were opened, a crowd of East Germans broke through.

The picnic represented the first massive exodus of East Germans to the west since the construction of the Berlin Wall in 1961 and was seen as a key factor in the fall of the wall itself three months later.

The picnic was organised by Hungarian opposition parties under the patronage of Imre Pozsgay, a prominent reformist member of the Communist party Politburo, and Otto von Habsburg, the president of the International Pan-European Union and son of the last Austro-Hungarian emperor.

The Hungarian government officially opened the border on September 11, 1989 after which an estimated 50,000 East Germans fled to Austria.