By Tadeusz Kolasinski

WARSAW (Reuters) - More than 200 monuments to Stalin's Red Army could be taken from towns across Poland and relocated on the site of a former Soviet military base under plans announced on Tuesday by a state-backed Polish historical institute.

At the risk of upsetting Moscow, the Institute of National Remembrance (IPN) proposes to house the so-called "monuments of gratitude to the Red Army" in a park in the former base at Borne Sulinowo, a small town 275 miles (440 km) northwest of Warsaw.

Poland, once a member of the former Soviet bloc but now a western ally in NATO, is still grappling with the legacy of war and communist rule and its relations with Moscow have been strained for some years.

All the same the IPN's plan is diplomatically sensitive, as Moscow protested strongly when one such monument was removed from the town of Pieniezno last year.

A spokeswoman for the Russian embassy in Warsaw, Ekaterina Glazova, said that Poland is obliged to protect all war memorials under a 1994 bilateral agreement with Russia.

Poland argues the agreement only relates to cemeteries, while Russia says it concerns all war memorials on Polish territory.

The Soviet Union lost more than 20 million people - more than any other country - in World War Two, and Russia considers the memorials a witness to its sacrifices in liberating Europe from the Nazis.

But many Poles resent them as reminders that Stalin and Hitler invaded their country simultaneously in 1939, and it remained under Soviet domination for more than four decades after the war until the overthrow of Communism in 1989.

Poland's new conservative government of the Law and Justice (PiS) party has been critical of Russian policy on Ukraine and is in favor of keeping EU sanctions on Moscow.

PiS has also reopened an investigation into the death of President Lech Kaczynski, the twin brother of current PiS leader, in a plane crash in Russia in 2010. PiS has never explicitly accused Russia of orchestrating the president's death, but has said the Kremlin benefited from the crash.

"The plan will include only monuments expressing the gratitude towards the Red Army, and it will not affect Soviet cemeteries," said Andrzej Zawistowski, director of the IPN's education department.

For that reason, the IPN said, it had not consulted Russia.

"The educational park will show these monuments within the right historical context," Zawistowski said. "Educational parks and institutions of this type exist equally in other states such as Lithuania, Hungary or even Russia."

The agency has cataloged 229 such monuments and it will now assist local authorities in delivering them to the park. But final decisions on whether to take them off the streets will be made by local councils.

(Reporting by Tadeusz Kolasinski; Editing by Richard Balmforth)