The towering Christmas tree in Dortmund's city centre has become a good-natured battle ground between rival football fans after a Schalke flag was spotted flying from the 45-metre giant.

Just 35 kilometres (21 miles) separates Borussia Dortmund's Signal Iduna Park stadium from Schalke's Veltins Arena in the Ruhr Valley and there is an intense rivalry between the neighbouring clubs.

Borussia won the Ruhr derby 3-2 at home a fortnight ago, but Schalke fans had the last laugh by flying their team's blue and white flag from the huge Christmas tree in the centre of Dortmund last weekend.

Anyone supporting Gelsenkirchen-based Schalke in Dortmund can expect trouble and the scaffolding company responsible for ensuring the 40,000kg (40 tonnes) tree stands safely has been told to pay a tongue-in-cheek fine to charity.

"I have informed the scaffolding company that I expect a penalty of 500 euros $532)," the tree's organiser Hans-Peter Arens told local newspaper WAZ.

The scaffolding firm responsible is based in Dortmund, but a large number of their employees hail from Bottrop, near Gelsenkirchen, -- deep in the heartland of Schalke fans.

A Dortmund supporter in the team of scaffolders restored order on Monday by flying a yellow and black Borussia flag from the tree and Arens said there will be no fine on this occasion.

"I can hardly hand out a fine when there is a Borussia Dortmund flag hanging from the tree," he quipped.