WARSAW (Reuters) - Human rights campaigners welcomed on Wednesday a report that prosecutors had charged the former head of Poland's intelligence service for helping set up CIA prisons for al Qaeda suspects in the country at the height of the U.S.-led "war on terror."

Daily Gazeta Wyborcza, a leading Polish newspaper, said on Tuesday that Zbigniew Siemiatkowski was charged as part of a classified investigation into the matter launched in 2008.

Despite reports by the European Parliament and the Council of Europe stating Poland and Romania hosted CIA detention centers, Polish officials have repeatedly denied the existence of such bases on its soil and say U.S. planes were only allowed to land for refueling.

At least two prisoners of the U.S. military jail in Guantanamo Bay in Cuba, Abd al-Rahim al-Nashiri and Abu Zubaydah, have said they had been held by U.S. agents in Poland.

Rights groups say detainees were kept there without court orders and often tortured.

"Poland deserves credit for this step, as the first European state to begin to deal with CIA torture on its own soil," London-based human rights group Reprieve said and urged Romania and Lithuania to follow Poland's lead.

Poland's smaller neighbor, Lithuania, was the first country in Europe to acknowledge it had worked with CIA in establishing two secret detention facilities in 2002-2006.

"Every state that has signed the (United Nations') Convention Against Torture has an obligation not just to prevent torture but to hold accountable officials who authorize or facilitate it," said Jameel Jaffer of the American Civil Liberties Union.

When contacted by Reuters on Wednesday, the U.S. embassy in Warsaw, the prosecutor's office and Siemiatkowski, who headed Poland's intelligence service in 2002-05, declined to comment on the report.

Poland has traditionally been one of the staunchest U.S. allies in Europe and has taken part in missions both in Afghanistan and Iraq.

The Polish constitution bans torture and imprisonment without court order. Politicians who authorize such activity could be tried in regular courts as well as the State Tribunal, a special court set up to try senior state officials.

"I suppose it may be true that the Polish secret service was making a base available and then was not really engaged in what was going on there," said Adam Bodnar of the Helsinki Committee for Human Rights.

"But for such an operation to go ahead smoothly various units must coordinate their actions and have approval from politicians at the highest level. Any intelligence head would not make such a decision alone," he said.

Leszek Miller, Poland's former leftist prime minister who held the post at times of the alleged CIA operations in Poland, reiterated on Tuesday that no secret prison operated in the country.

(Reporting by Gabriela Baczynska; Editing by Karolina Tagaris)