* Pipeline issue already led to dismissal of treasury minister
* Poland fears Ukraine being sidelined by Russia's plans
* CEO says decision not based on merits of her work (Adds CEO comments, share price, details)
WARSAW, April 29 (Reuters) - Polish gas monopoly PGNiG fired its chief executive on Monday after she failed to stop a joint venture with Russia's Gazprom from opening the way for a new Russian pipeline cutting out Poland's ally Ukraine.
Prime Minister Donald Tusk had already dismissed his treasury minister less than two weeks ago after learning from media reports that the venture had signed a memorandum to build a segment of the Yamal-Europe gas pipeline via Belarus and then Poland.
PGNiG, in which Poland holds a 72-percent stake, said in a statement its supervisory board had lost confidence in the CEO, Grazyna Piotrowska-Oliwa, one of only a handful of top female executives at the biggest state companies.
Piotrowska-Oliwa said all her actions related to the memorandum on the new Russian pipeline were lawful and accordance with her responsibilities to shareholders.
"I don't treat today's decision to dismiss me as based on the merits of my work as the chief executive," she said.
The board also dismissed her deputy, Radoslaw Dudzinski, who was not immediately available to comment.
After disputes with Kiev over gas prices that have led to repeated winter-time disruptions of Russian pipeline gas exports across Ukraine in recent years, Russia has set out to bypass the country with pipelines taking other routes to European markets.
Poland, the European Union's largest eastern member, sees itself as Ukraine's strategic partner and is a leading advocate of bringing it closer to Europe.
Poland fears Russia would be able to cut off gas supplies to Ukraine much quicker with the alternative export routes.
Piotrowska-Oliwa was at the helm when Gazprom, Russia's pipeline gas export monopoly, agreed in November to reduce prices on oil sold to PGNiG, which had been losing money on every barrel it was importing from the east.
Mainly as a result of the deal, PGNiG shares surged 46 percent during her tenure.
PGNiG shares closed 1.9 percent lower on Monday compared with a 0.4 percent rise of Warsaw's main index. (Reporting by Chris Borowski; Editing by Anthony Barker)