Before the swearing-in, Parliament endorsed the report by the Credentials and Privileges Commission that unanimously decided two days ago that all legal requirements were met for the activation of Sanader's term as an MP.

Sanader, who abruptly resigned as Prime Minister at the beginning of July last year, delayed his term as a parliamentary deputy on July 9, 2009, citing personal reasons, and filed a request for its activation on October 14, 2010.

He replaced Zeljana Podrug, a member of the ruling Croatian Democratic Union (HDZ) who had deputised for him while he had served as Prime Minister.

Sanader won a term on the HDZ slate in 2007's parliamentary election. Since he was expelled from the party early this year, he will serve as an independent MP.

Sanader told reporters after the swearing-in ceremony that "unbelievable and undemocratic developments" had forced him to return to politics.

The former PM said he hoped that together with his colleagues in Parliament he would get to the bottom of such developments, which he added were "absolutely incompatible with European democratic standards."

He would not specify which developments he had in mind.