As the economist now believes, Berlin should do much more to diversify energy supplies.

"If we had known then what we know now, we would certainly have acted differently," he said.

At the same time, Roller acknowledged that Russia's plentiful and low-cost energy exports gave a powerful boost to the German economy and helped ensure growth for ten consecutive years.

In October last year, Angela Merkel defended her decision to buy large volumes of natural gas from Russia.