However, Saturday's ruling, which highlighted widespread fraud in the elections, has no legal force as parliament claims no jurisdiction over the validity of elections.
But it provided a further boost for liberal opposition candidate Viktor Yushchenko in his bid to overturn Yanukovich's victory in the 21 November election.
The declaration was issued two days before a Supreme Court case examining Yushchenko's complaints of poll irregularities and as the European Union said it wanted to see new elections.
The overwhelming vote, which also expressed no confidence in the Central Election Commission overseeing the election, was taken a day after the rivals agreed at a meeting with mediators to meet regularly to resolve their differences.
"This was an extraordinarily difficult day," parliament speaker Volodymyr Lytvyn told Interfax Ukraine news agency. "Political leaders will now have to objectively analyse the possibilities and limits of compromise."
No date for a new election was suggested. Yushchenko has already said he wants a new vote on 12 December.
Lytvyn told deputies to prepare for a revision of legislation to take account of proposed changes in electoral procedures.
Viktor Yushchenko says he wants
a new vote on 12 December
Olexander Moroz, an Yushchenko ally, said the opposition could try to censure Yanukovich's government next week, though that also would be symbolic.
Commission head Serhiy Kivalov dismissed the assembly's criticism, saying opposition monitors had signed all documents.
"They felt there was no point in a challenge using legal means," he told Inter television. "They used politics instead."
The two rivals went to ground after parliament's decision.
Yanukovich failed to turn up at a planned news conference.
Viktor Yanukovich failed to turn
up at a news conference
Yushchenko declined to make his usual daily appearance before the tens of thousands of supporters who have packed Kiev's main square since he claimed he was cheated out of the presidency.
A party atmosphere prevailed in Independence Square and Kiev's tree-lined main street, Khreshchatyk, with vast crowds waving flags and wearing trademark orange gear strolling well into the night past a tent camp and improvised food stands.
Opposition backers were certain to pour back into the square on Sunday in response to Yushchenko's plea to remain there.
Western countries have called for a revision of the results, with the European Union advocating a new election.
The election placed voters before two visions of Ukraine - Yushchenko called for gradual integration with the West while Yanukovich saw closer ties with Russia as the key to prosperity.
The split also raised tensions between the West and Russia, which backed the prime minister as best-placed to defend its interests.
And it plunged the country deeper into a longstanding divide between the nationalist west, supporting Yushchenko, and the industrial Russian-speaking east, solidly behind the prime minister.
Officials in eastern regions, furious at the challenger's attempts to overturn the outcome, were to meet on Sunday, with talk circulating of a referendum on "autonomy" from Kiev.
A large demonstration backing the prime minister surged through the mining town of Donetsk on Saturday.
Also to come, on Monday was a Supreme Court ruling on a case submitted by Yushchenko on cheating in the second round.
The court last week forbade publication of the count handing victory to the prime minister - forcing postponement of his inauguration pending resolution of legal issues.