Venezuela's electoral body has announced it will carry out a full audit on all the votes cast in Sunday's disputed presidential poll.
The announcement comes hours ahead of President-elect Nicolas Maduro's inauguration.
Opposition leader Henrique Capriles has challenged Mr Maduro's narrow victory, calling for a vote-by-vote recount.
The decision to audit all the paper receipts of electronic votes is seen as a major concession to the opposition.
The electoral authorities had earlier audited 54% of the vote and said this showed that Mr Maduro, the chosen successor of the late President Hugo Chavez, had won a slim majority.
The official count indicates Mr Maduro won by a narrow margin of about 1.5%, winning 50.8% of votes to Mr Capriles's 49.0%
Mr Capriles said he believed the crucial votes that cost him the presidency are among the unaudited 46% of the vote.
He said there were more than 3,000 incidents from Sunday's poll that needed to be examined.
Correspondents say the announcement comes as a surprise to many after the National Electoral Council initially said the the results, which it announced on Sunday night, were "irreversible".
The council's president, Tibisay Lucena, told AFP news agency that the expanded audit was not a recount but would cover all ballot boxes not audited on election day by reviewing a sample two-thirds of them over the next month.
Venezuela uses electronic voting machines which register an elector's decision and then emit a printed receipt for the voter to deposit into a sealed ballot box. For the audit, the receipts will be compared with the electronic tallies, to check for any irregularities.
Russian President Vladimir Putin and Cuban leader Raul Castro were among the first heads of state to congratulate Mr Maduro on his win.
The governments of Brazil, Peru, Ecuador, Colombia, Bolivia and Argentina, among others, have also voiced their backing for Mr Maduro's victory.
But the US has so far refused to recognise Mr Maduro's win, calling for an audit of the results.
Secretary of State John Kerry said the US was not yet ready to validate the results of Sunday's poll.
Several opposition-led protests erupted across the country after the official results were announced on Sunday.
But Mr Capriles has urged his supporters to back down from confrontation and instead engage in peaceful rallies, play music and bang on pots in protest.
Mr Maduro is scheduled to be sworn in at 15:30 GMT.