By Sergio Goncalves and Andrei Khalip

LISBON (Reuters) -Portugal's parliament rejected on Tuesday a motion of confidence in the centre-right minority government, provoking its collapse after only 11 months in office that is likely to trigger an early election, the country's third in just over three years.

Lawmakers voted 142-88, with zero abstentions, against the motion that was presented last Thursday by Prime Minister Luis Montenegro, after the opposition questioned his integrity over the dealings of a consultancy firm he founded which is now run by his sons.

His administration now assumes a caretaker role.

Montenegro has denied wrongdoing or any ethical shortcomings by the firm, which has contracts with private companies.

"The insinuation that I mixed my business and political activity is completely abusive, and even insulting. A repeated falsehood does not become the truth, but it contaminates the political environment... this is what populism feeds on," he told parliament before the vote.

It is now up to President Marcelo Rebelo de Sousa whether to call a new parliamentary election after a round of consultations that are likely to take place in the next few days. He has said a new ballot could be held in mid-May.

STABILITY SEEN ELUSIVE, VOTERS WEARY

Analysts see an early ballot as all but inevitable but see no strong mandate for any political force emerging from it.

Meanwhile voters are already showing election fatigue and disillusionment with politicians.

"This seems like a joke, no one understands why there's a new election so soon. Politicians blame each other, but all of them are being irresponsible," said Joao Brito, 70-year-old retired civil servant in downtown Lisbon.

Political scientist Adelino Maltez of Lisbon University pointed to opinion polls showing very little change in voter preferences from the March 2024 election, in which Montenegro's Democratic Alliance (AD) won by a mere 1,500 votes, securing 80 seats in the 230-seat house.

The AD and the Socialists, who now have 78 seats, are neck and neck in most surveys.

"The problem is that the new election will not be conclusive... The AD and the Socialists are tied. It is a situation that will be difficult for them to navigate," Maltez said, seeing a centrist pact between Montenegro's Social Democrats and the Socialists as the only solution.

The two main rivals only had such an accord in parliament once, in 1983-1985.

"If they don't do it, it will be more of the same instability," Maltez said, seeing the rivals' programmes as largely compatible.

(Reporting by Sergio Goncalves, Writing by Andrei Khalip, editing by Aislinn Laing, Christina Fincher and David Latona)