At least three people taking part in UN-led Libyan peace talks were bribed for votes, researchers said in a report for the Security Council.
Prime minister-designate Abdul Hamid Dbeibah was chosen in February at the Libyan Political Dialogue Forum launched in Tunisia in November, in the UN's latest bid to salvage the country from a decade of conflict and fragmentation.
In the report, to be presented to the Security Council in March, UN researchers said two people taking part "offered bribes of between $150,000 and $200,000 to at least three participants if they committed to vote for Dbeibah as PM".
The report was prepared by UN officials given the responsibility of examining breaches of an international arms embargo to Libya.
The researchers found that one delegate "erupted in anger in lobby of the Four Seasons Hotel in Tunis on hearing that some participants may have received up to ... $500,000 for their Dbeibah votes, whereas he had only received $200,000".
One participant in the talks said he saw the scene, and was angry about "unacceptable corruption at a time when Libya is going through a major crisis".
The Tunisia talks are trying to form a transitional administration to lead the country towards elections set for December.
In February, the 75 people chosen by the UN to represent a wide spectrum of Libyan life selected an interim administration led by billionaire Mr Dbeibah and a three-member presidential council.
Mr Dbeibah must now win a vote of confidence in his government from the Libyan Parliament by March 19.
He said on Thursday that he had submitted a "vision" for a Cabinet line-up but that the names of proposed ministers would be disclosed in Parliament during the confidence vote.
Several Libyan organisations in November demanded an investigation into allegations of corruption in choosing future officials.
The UN's interim Libya envoy at the time, Stephanie Williams, said she had launched an investigation into allegations of bribery.
In a letter dated February 20, forum participants Sayida Yaacoubi and Azza Assid called on Ms Williams's permanent successor, Jan Kubis, to make the report public, saying the claims were an affront to their "dignity, honour and transparency".