Centre-left leader Romano Prodi, who claimed victory in Italy's general election, has poured scorn on PM Silvio Berlusconi's allegations of fraud.
Silvio Berlusconi remains caretaker PM for several weeks
Mr Prodi said Mr Berlusconi's call for a review of votes was "extraordinary" because the prime minister had "made an electoral law for himself".
Mr Berlusconi wants the records of all 60,000 polling stations to be checked.
Mr Prodi was declared the winner of both houses of parliament - but by an extremely narrow margin.
Prime Minister Berlusconi, heading a centre-right coalition, voiced concern that there were more than a million spoilt and blank ballots in the election.
He said there had been "much fraud" in the poll, held on Sunday and Monday.
The United States said it would refrain from congratulating Mr Prodi until the end of the electoral process, which is likely to go to court.
Review of ballots
Election officials are now routinely checking about 43,000 contested ballot papers. The outcome of their check is expected later on Thursday or Friday.
Mr Berlusconi's government had pushed through an electoral reform last year which brought back full proportional representation. Critics said it increased the risk of unstable governments, characteristic of post-war Italy.
Mr Prodi, previously an Italian prime minister and president of the European Commission, said his victory would not be reversed.
"We won and it is useless for Berlusconi to try to delay it. Berlusconi must go home," he told supporters in his home town Bologna on Wednesday.
Mr Prodi said he had already begun talks on forming a new administration and tackling the Mafia would be an absolute priority.
However, no administration can be sworn in for another month, until parliament has elected a new president.
The current head-of-state, Carlo Azeglio Ciampi, is due to step down and he has made it clear he wants his successor to preside over the change of government.
The new parliament is due to convene on 28 April and is not scheduled to vote on a successor to Mr Ciampi until 12-13 May.
Official results showed Mr Prodi had won just enough seats to control the Senate (upper house) after having already won a lower house majority.
The count gave Mr Prodi 158 Senate seats, against 156 for Mr Berlusconi.
Mr Prodi's coalition is thought to have won the vote in the Chamber of Deputies (lower house) by just 25,000 votes.
The electoral changes meant whoever won the lower house would automatically get a working majority there, even if the margin was small.
The interior ministry insists the results must still be confirmed by Italy's highest court, and that parliament's election committees would have to rule on any challenges.
The final results came after a nail-biting night of conflicting forecasts, based on exit polls and partial counts, which variously put the coalitions of Mr Prodi and his rival ahead.
The leader of the observation team from the Organization for Security and Co-operation in Europe, Peter Eicher, said there appeared to be irregularities including "a very large number of blank, invalid or contested ballots".
He did not accuse either side, but said he also had misgivings about the late adoption of new electoral laws and unbalanced coverage by parts of the Italian media.