By Marcin Goettig

WARSAW (Reuters) - Polish prosecutors said on Thursday they had begun investigating whether the head of the constitutional tribunal exceeded his authority by not allowing three judges chosen by the ruling parliamentary majority to rule on cases.

The inquiry marks another stage in the conflict between Poland's eurosceptic government and its top court. The conflict has already soured investor sentiment toward Poland, eastern Europe's biggest economy.

"This investigation is at a very early stage. We are gathering documents and are exchanging letters with the constitutional tribunal," said Ireneusz Kunert, a spokesman for the regional prosecutor's office in the city of Katowice.

The head of the constitutional court, Andrzej Rzeplinski, told state news agency PAP the investigation was as an "incompetent attempt" to encroach upon the independence of the judiciary.

Earlier this year, prosecutors in Warsaw declined to investigate whether the refusal of the government to publish some verdicts of the constitutional court was legal.

"Perhaps this investigation is a form of exerting pressure on the head of the tribunal, but I doubt it will impact actions of the court's head," said Piotr Kladoczny of the Helsinki Foundation for Human Rights.

Government spokesman Rafal Bochenek said that the decision of the prosecutor's office was independent.

Since winning an outright parliamentary majority last year, the ruling Law and Justice (PiS) party has passed several changes to the constitutional court's operational rules. The court ruled some of these change were illegal, but the government ignored the verdicts.

Parliament also scrapped last year the previous parliament's nominations for five of the court's 15 judges and put forward its own candidates, which were immediately sworn in by President Andrzej Duda, an ally of PiS.

The tribunal later ruled that three of the nominations of the previous party were legal and two illegal. PiS representatives said then that the process of the five new appointments had already been completed.

Critics say the government's changes to the tribunal had undermined democratic standards and are part of a broader push to seize more control over state institutions, charges denied by PiS.

(Additional reporting by Pawel Sobczak; Writing by Marcin Goettig; Editing by Larry King)