The official, confirming comments made by the head of the Hellenic Capital Market Commission, said there were indications that rules had been violated in about 150 cases.

Short-selling involves investors borrowing shares to sell on the market and later buying them back at a lower price for a profit.

In this instance, the investigation is focussing on suspicion that some funds or individuals participating in rights issues offered by the top four Greek banks - National Bank (>> National Bank of Greece), Alpha Bank (>> Alpha Bank S.A.), Piraeus (>> Piraeus Bank SA) and Eurobank (>> Eurobank Ergasias SA) - were shorting bank stocks days before they were due to receive their allotted shares from the issue, the official said.

The regulator had been tipped off by heavy trading volumes and a sharp fall in bank share prices just before the new shares began trading on the market, the official said.

The investigation is expected to conclude in January, the official said.

(Reporting by Lefteris Papadimas, writing by Renee Maltezou, editing by David Evans)