KIEV, May 25 - The Arbitration Institute of the Stockholm Chamber of Commerce may issue a head-note ruling on the gas supply contract between Naftogaz Ukrayiny and Russia's Gazprom before the end of May, and then it can provide a detailed decision, Naftogaz Chief Commercial Officer Yuriy Vitrenko told reporters.

"We expect the decision from Stockholm before the end of May so far, which will be a formal legally binding decision," he said.