Culture Minister Piotr Gliński’s announcement that the new museum was established follows a Wednesday Supreme Administrative Court decision which overturned an earlier ruling that blocked the merger.
The ministry earlier said that two museums devoted to the same historical period in one city were economically and logistically unjustified.
However, some Polish media suggested that the decision is meant to serve the governing Law and Justice (PiS) party’s political objectives by supporting an “approved historical message”, and that the merger is only a façade.
The existing WWII museum, which opened late last month, cost nearly PLN 450 million (EUR 105 million) to build and has already had some 20 million visitors.
It is the largest of its kind in Poland and is a state-of-the-art facility focused on the fates of both Polish and other civilians in the war.
It claims to be a “history lesson for foreigners” and was consulted with experts including famous British historian Norman Davies.
But it has been criticised by PiS for “not being patriotic enough”, according to media reports.
Davies told UK paper The Observer that “the Law and Justice government does not want a bunch of foreign historians to decide what goes on in ‘their’ museum.’’
Meanwhile, a merger with the Westerplatte museum could allow the government to appoint its own director to the new facility and rewrite its narrative.
Gliński earlier told the Rzeczpospolita daily that: “The content of an exposition of a museum as important as the WWII Museum is neither a matter of the tastes of even great Polish and foreign historians nor the minister”.
“The content should express the basic theses and directions of historic policy,” he added.
Gliński on Thursday also announced that Karol Nawrocki will be the new museum's director.
Nawrocki said he did not accept the position to make changes.
But "every museum in the world changes," he added.
He said he is open to the opinions of experts and the public opinion.
"Maybe some of them will decide that some parts of the exhibition be changed," Nawrocki said.
He also requested that doners leave their artefacts at the museum where they will "remain safe".
The Westerplatte museum was announced by the government in December 2015, just two months after PiS won parliamentary elections, and exists only on paper.
The Museum of the Second World War was announced in 2012 by then-Prime Minister Donald Tusk, a co-founder of the opposition Civic Platform (PO) party. (vb)