The Institute of National Remembrance (IPN) oversees and analyses Poland's communist-era files but can also prosecute those it believes committed "crimes against the nation", by for example collaborating with the secret police.
The head of IPN, Janusz Kurtyka, was killed along with his close political ally President Lech Kaczynski and 94 other mostly senior officials in a plane crash in Russia on April 10.
Kaczynski, a eurosceptic right-winger, had been expected to veto the centrist government's bill, which changes the rules for appointing the head of the IPN. Acting President Bronislaw Komorowski, who took the helm on Kaczynski's death in his capacity as speaker of parliament, is the candidate of Prime Minister Donald Tusk's ruling Civic Platform (PO) in a presidential election now set for June 20.
"Today I signed nine bills including... the one on the IPN," Komorowski told reporters.
Critics accuse the IPN of conducting political witch-hunts instead of engaging in objective academic research.
The current IPN management, mostly appointed by Poland's main opposition Law and Justice (PiS) party when it was in government, has taken steps to name a replacement for Kurtyka, even though parliament had already approved the bill at the time of the crash and was awaiting the president's decision.
But Komorowski said that process, taken under old rules, was no longer valid.
The new law sets up a special body of academics that will propose a candidate to head the IPN. Parliament, where Tusk's PO now has a majority, would have to approve the nominee. The new academic council can also set research areas for the IPN.
"Everything points to the fact that, given the time needed for the bill to take effect and to create the new council, the current leaders of the IPN will stay in place for some while," Komorowski said.
The new law also grants wider public access to the files.
Komorowski said parliament was working on a further change to IPN regulations aimed at clarifying who takes over the body if its president dies in office. The IPN's deputy head, Franciszek Gryciuk, has headed the IPN since Kurtyka's death.
The PiS-led government of 2005-2007, led by Kaczynski's twin brother Jaroslaw, encouraged IPN prosecutions against alleged communist-era collaborators as part of its "moral revival" campaign aimed at removing from public life people who had held senior posts under the old regime that collapsed in 1989.
The IPN sparked an outcry when it accused Lech Walesa, onetime leader of the pro-democracy Solidarity trade union and Poland's first post-communist president, of cooperating with the communist regime in 1970, a claim Walesa strongly denies.
Tusk defended "national treasure" Walesa. Lech Kaczynski, himself a onetime Solidarity activist who later broke with Walesa, backed the IPN in the dispute.
