Portugal’s president has invited Luís Montenegro, the leader of the Democratic Alliance (AD), to try to form a minority government after a long-awaited count of overseas votes confirmed a narrow election victory for the centre-right bloc.

Montenegro was summoned to the presidential palace in Lisbon shortly after midnight on Thursday where President Marcelo Rebelo de Sousa, who has spent over a week consulting party leaders, formally nominated him to head the government.

“The AD won the election … [so] the president … decided to nominate Luís Montenegro as prime minister,” the president’s office said in a statement.

Overall, the AD won 80 seats in the 230-seat legislature, which is expected to return next week, followed by the Socialists at 78 seats and the far-right Chega party, which was founded just five years ago, with 50.

After meeting Rebelo de Sousa for a first time on Wednesday afternoon, Montenegro said that on behalf of his party he had “expressed our willingness to take on the leadership of the government and to be appointed prime minister”.

Montenegro has pledged not to enter a coalition or even an informal alliance with Chega – his only route to a majority government – and must now try to agree with the Socialist party (PS) the outline of a legislative programme the centre-left party will back in parliament.

On Tuesday Rebelo de Sousa met the Socialists’ new leader, Pedro Nuno Santos, who promised the party would be a “stable, strong and solid” opposition, but also that it would be a “responsible opposition” that was “open to agreements”.

The PS would not back proposals with which it did not agree, he said, but nor would it oppose “where there are common points of view”, such as on the need to boost pay for public sector workers including teachers, health professionals and police.

Chega, which emerged as potential kingmaker after campaigning on a platform calling for stricter controls on immigration and tougher measures to fight corruption, has demanded a government role in exchange for supporting an AD-led administration.

The populist party’s leader, André Ventura, warned after his meeting with Rebelo de Sousa on Monday that if AD continued to reject a coalition with Chega, voters would inevitably blame the centre-right party for any political instability that ensued.

“We are continuing to put in all our efforts … to reach an agreement that will ensure the country’s stability,” Ventura said. “If there is no government agreement, the AD will be responsible for the instability that will result.”

António Costa Pinto, a political scientist at the University of Lisbon, told Agence France-Presse that a minority administration would “not necessarily” be unstable because “none of the actors has an interest in triggering a crisis”.

Another political scientist, José Adelino Maltez, told the Diario de Noticias newspaper that “in political terms there is no crisis. Seventy percent of the AD programme is 70% of the PS program. There is great stability on all the essential objectives.”

However, Montenegro is already coming under heavy pressure from a small but determined group of MPs from his Social Democratic party (PSD) who argue that a stable majority government in alliance with Chega is the only responsible course.

Without it, Montenegro will be obliged to try to pass legislation on a case-by-case basis, and his government could face a survival test as early as this autumn when it tries to draw up the 2025 budget. A rejected budget could lead to a new election.