Borussia Dortmund chief executive Hans-Joachim Watzke has slammed the mega spending of Champions League opponents Manchester City.

Watzke says City should be thrown out of the Champions League if they try to bend the new financial fair play rules.

"UEFA must find the thin line between sponsorship and excessive back-door funding - they must show strength to expel big clubs," said Watzke, whose club is 51 per cent owned by its supporters and has a strict policy not to allow spending to exceed their income.

"No tycoon should be allowed to pump crazy money into a club with sponsorship from five companies he controls. If that happens, financial fair play will fail."