Kiev (AFP) -- Former Ukranian premier Yulia Tymoshenko challenged Monday the government's legitimacy, saying Prime Minister Viktor Yanukovich had illegally taken his post without resigning his parliamentary seat.

Speaking at her first press conference in a month, Yanukovich's rival cited a law requiring all legislators to resign their seats within 20 days of being nominated to a cabinet post.

According to the Ukranian parliament's official website, the pro-Russian Yanukovich and 10 of his ministers -- all appointed one month ago -- still hold their electoral seats.

As a result, any action or decision taken by the cabinet is "illegitimate", Tymoshenko argued, calling on pro-western President Viktor Yushchenko to put an end to the government's "illegal activities".

Failing that, Tymoshenko threatened to take the case to the constitutional court.

Yushchenko had appointed Tymoshenko, his partner in Ukraine's "orange revolution", as prime minister in February 2005 but sacked her just seven months later after disagreements over privatization and her economic interventionism.

After months of post-election political turmoil, Tymoshenko's parliamentary block boycotted Yanukovich's confirmation as premier on August 4, having labelled his appointment as "an act of capitulation" by her fellow orange revolutionaries.