ROME, Dec. 5 (Xinhua) -- Italy's Prime Minister Matteo Renzi will resign after the 2017 draft budget is approved, the Italian president said on Monday.

The outgoing prime minister formally handed in his resignation to president Sergio Mattarella in the evening, after a major constitutional reform backed by his cabinet was rejected by 59 percent of voters in a referendum on Sunday.

However, Renzi agreed to meet a president's specific request to postpone his stepping down.

"The President of the Republic, given the need to complete the parliamentary proceeding to approve the budget law, has asked the President of the Council to postpone his resignation until such commitment is fulfilled," Mattarella said in a statement.

The country's budget has to be approved by the end of 2016, and still lacks the green light by the senate.

The president's intent to delay the political crisis until major pending issues were solved was quite clear since after he held informal talks with Renzi earlier on Monday.

After the talks, in fact, Mattarella had issued a statement stressing the country's institutions should honor all necessary deadlines.

"The high turnout in yesterday's referendum is proof of a solid democracy, of a country passionate and capable of an active participation," he said. "The institutions must in any case guarantee the fulfilment of all commitments and tasks awaiting us, ensuring an adequate answer to the problems of the moment."

The Upper House's final vote on the 2017 draft budget might take place as soon as Friday, or early next week, Ansa news agency said, citing government sources.

After such step, and Renzi's official resignation, the president will launch a round of consultations with party leaders to appoint a new prime minister backed by a sufficiently solid parliamentary majority.

Economy Minister Pier Carlo Padoan and president of the Senate Pietro Grasso were seen as possible candidates to replace Renzi, according to local media reports.

Some opposition forces have called for immediate elections, but this option appeared very unlikely. In fact, the country has another priority to deal with: new electoral rules.

The next cabinet will have to draw a new electoral law, or amend the one approved in 2015, since the latter already foresaw a demoted senate as proposed in the constitutional reform just rejected by citizens.

Then, Mattarella might ask the cabinet to stay in power until the natural end of legislature in 2018, or he would call early elections to be held in spring 2017.

The constitutional referendum held on Sunday proved a key test for Renzi's center-left government.

Renzi had strongly advocated the reform, claiming it would help streamline Italy's laborious law-making process and increase political stability. The major amendment proposed would have demoted the senate, leaving to the House of Deputies only the power to bring down the cabinet.

The No camp, which was led by the populist Five Star Movement (M5S) and included center-right forces, leftist forces, and a minority of Renzi's own center-left Democratic Party, claimed the reform would have given too much power to the executive branch.