An investor has sued Deutsche Bank over the price it offered shareholders in the retail bank, arguing the two banks had drafted a blueprint of the takeover before the financial crisis and Deutsche should therefore pay investors the price it paid for an initial minority stake.

While lower courts found no wrong-doing by Deutsche Bank, the Supreme Court said it was possible that Germany's flagship lender may have acted in concert with Postbank, the court said in a statement on Tuesday.

It has asked the appeals court to check if such acting in concert took place, in which case Deutsche Bank might have to offer compensation to investors.

In September 2008, Deutsche Bank bought a Postbank stake of 29.75 percent for 57.25 euros (45.35 pounds) a share.

In January 2009, when the financial crisis had sent stock prices on a world wide slide, Deutsche Bank upped its stake to 48 percent, triggering a mandatory bid. It ended up offering 25 euros a share to other investors. In several further steps, Deutsche Bank later bought the remaining shares.

The investor, Effecten-Spiegel, which also produces stock market reports, accepted the 25 euros per share offer and sold its 150,000 Postbank shares for 3.75 million euros.

It then sued Deutsche Bank seeking an additional 4.8 million euros, arguing the steps to take full control of Postbank had already been planned in 2008 and the banks therefore acted in concert to carry out the transaction.

(Reporting by Arno Schuetze; Editing by Mark Potter)