MADRID, Nov 6 (Reuters) - Spain's Supreme Court said on Tuesday that customers and not banks will have to pay for stamp duty on mortgages from now on, reversing a ruling from mid-October and sparing banks from billion of euros in potential costs.

The unexpected ruling, which adheres to an even earlier ruling from February, is likely to lead to a relief rally amongst Spanish banks on Wednesday. The back-and-forth over the decision had prompted Spanish lenders to demand more predictable rules. (Reporting By Jesus Aguado; Editing by Sonya Dowsett)