Polish prosecutors have obtained a top secret memorandum from either late 2005 or early 2006 showing that a CIA prison existed in Poland earlier in the decade, according to respected Polish daily Gazeta Wyborcza.

The Warsaw newspaper quoted sources saying that they had seen the document but did not explicitly say that the prison served as a holding center for al-Qaeda terrorist suspects as has been alleged in the American media.

The Gazeta reported that according to the document the secret prison was set up after the US and Poland signed an accord in 2002 to combat terrorism.

CIA prison allegations surfaced in 2005

Polish prosecutors had been investigating long-running allegations of a secret CIA jail to interrogate top al-Qaeda operatives near Szymany in northeast Poland.

In 2006, the head of the secret services had presented the two-page document to two ministers from the then ruling conservative Law and Justice Party and Poland's chief prosecutor Janusz Kaczmarek at the time, according to Gazeta.

Kaczmarek confirmed the meeting with the ministers, but refused to comment on what transpired. The ex-ministers declined to speak with the newspaper.

In the past Polish officials have consistently rejected media claims that Warsaw allowed the CIA to run a secret prison for captured Islamic radicals.

Polish top aide denies knowledge of memo

But on Friday, a top advisor to Prime Minister Donald Tusk did confirm that prosecutors were investigating the allegations, although he denied any knowledge of the secret memorandum.

"I am not familiar with such a note and I don't think Prime Minister Donald Tusk is either," said Tusk's aide Slawomir Nowak in an interview with Poland's Tok FM radio.

"But the premier asked the justice minister to clarify this matter and the country's prosecutor's office is investigating the potential existence of the CIA prison."

Back in 2005, the Washington Post broke the story that CIA prisons existed in Europe.

Close cooperation between US and Polish secret services

"There definitely was cooperation between Polish and American secret services," a source close to the US intelligence agency told news agency Reuters. "But whether there was torture at the base, hopefully we will learn about that soon."

Rumors circulated in the US and foreign media that the base was operational between 2002 and 2005 at a time when Poland was governed by the left-leaning governments of Leszek Miller and Marek Belka and then a rightist administration under Kazimierz Marcinkiewicz.

All three former leaders have denied any knowledge of such a prison or base, and Tusk's center-right government has until now also played down speculation of its existence.

"I hope this will not be confirmed," Nowak said to Reuters. "It would not only have serious consequences inside the country but would also take a very serious toll on the international scene. This has to be investigated very carefully, without emotions."

Since the Washington Post story emerged, separate reports by the Council of Europe, the European Parliament and the European Commission have confirmed the existence of CIA prisons in Poland and Romania.