The report by the U.N. Panel of Experts that monitors the ban on transfers of weapons to Libya says companies controlled by Mr. Prince provided three aircraft to assist in sending helicopters and military contractors to help Russian-backed Libyan warlord Khalifa Haftar in 2019.

The plan to send Western mercenaries to Libya developed as foreign weapons and fighters poured into the country in 2019 and 2020 from a variety of outside powers, including Russia, Turkey and the United Arab Emirates, deepening a conflict that has been ongoing since 2014, the report says, according to the diplomat.

Mr. Prince is likely to be referred to the U.N.’s Sanctions Committee, which could order a freeze on his assets or a travel ban, according to the New York-based diplomat and a former official with knowledge of the situation. The permanent members of the Security Council, including the U.S., Russia, or China, could veto any potential sanctions against Mr. Prince, who has had dealings with all three countries.

“Erik Prince had absolutely nothing to do with any operation in Libya in 2019, or at any other time,” a spokesman for Mr. Prince said in an email.

A U.N. spokesman said the organization had no specific comment on the Panel of Experts report.

“It is incumbent on our member states to ensure that the sanctions are respected and enforced,” said U.N. spokesman Stephane Dujarric De La Rivière.

The report itself has been finalized and submitted to the U.N.’s headquarters in New York. It is unlikely to be altered before it is released to the public in the coming weeks, according to diplomats.

Mr. Prince, a former Navy SEAL, came to prominence during the Iraq war, when Blackwater provided private security guards to U.S. officials and contractors working for the company shot dead more than a dozen Iraqi civilians in a 2007 mass killing in Baghdad. Blackwater has since changed its name to Xe Services and later Academi.

Mr. Prince’s financial and political ambitions rose because of his close relationship to the Trump administration. Mr. Prince is the brother of Mr. Trump’s former education secretary, Betsy DeVos. In December, Mr. Trump pardoned the four Blackwater guards accused in the 2007 killings.

According to the diplomat, the forthcoming U.N. report says companies controlled by Mr. Prince sold three aircraft to people who sent Western mercenaries and military hardware to aid Mr. Haftar in the opening months of the commander’s failed assault on Libya’s internationally recognized government in Tripoli. Launched in April 2019, Mr. Haftar’s attack on the capital plunged Libya into its worst fighting since the armed rebellion that overthrew Col. Moammar Gadhafi in 2011.

According to the diplomat, the U.N. panel’s report says that firms controlled by Mr. Prince sold three aircraft through a series of shell companies to a Dubai-based company, Lancaster 6, which sent helicopters and a group of Western mercenaries to Libya to support Mr. Haftar. The plan unraveled, and the fighters left Libya.