GDANSK, Oct 18 (Reuters) - Poland's biggest bank PKO Bank Polski sold 14.2 million shares of oil company PKN Orlen at 50.50 zlotys ($10.39) per share, via an accelerated book building process launched on Monday, the bank said in a statement on Tuesday.

At the end of September, PKO Bank Polski had signed a deal with Poland's state treasury to buy the stake in PKN Orlen. The deal was linked to PKN Orlen's plan to merge with gas company PGNiG.

On Sept. 30, PKO Bank Polski and the oil company had also signed a total return swap agreement that was meant to limit the bank's exposure to Orlen's share price fluctuations.

PKO Bank Polski said in its statement on Tuesday that the "economic result of the transaction will be impacted by the conditions set in the return swap deal."

At the end of Tuesday's session, PKN's shares closed down 4.5% at 51.68 zlotys ($10.63) each. ($1 = 4.8598 zlotys) (Reporting by Mateusz Rabiega and Maria Gieldon. Editing by Jane Merriman)