Portugal's stock market regulator on Friday suspended trading in shares of Portugal Telecom SGPS SA until the firm discloses "relevant information" about the sale of an operating unit to France's Altice SA.

The regulator, CMVM, said it had asked Portugal Telecom several times since Dec. 22 to provide more details about a Jan. 12 shareholder meeting to approve the sale but hadn't received a response.

PT is in the process of merging with Brazilian phone company Oi. The combined group is planning to sell its operations in Portugal to Luxembourg-based Altice for EUR7.4 billion ($9.13 billion).

The suspension followed PricewaterhouseCoopers' publication of findings from an investigation into failed investments made by Portugal Telecom in units of collapsed Portuguese financial conglomerate Esprito Santo.

PWC said in a 31-page report that PT had failed to perform any risk analysis before purchasing hundreds of millions of euros worth of bonds issued by the Espirito Santo units.

The report said PT had made consecutive purchases of bonds issued by units including Rio Forte Investments SA "immediately upon the termination of another investment." This successive rollover of investments may have delayed the defaults of Rio Forte and another unit of the conglomerate, Espirito Santo International. The two units defaulted in July 2014.

The auditor said it based its conclusions on documentary evidence and interviews with former and current executives of PT and Espirito Santo. It said it couldn't establish who at the phone company had authorized the investments. Between 2010 and 2014, the report said, purchases of securities issued by the Espirito Santo group were never subject to approval or consideration by PT's board of directors, executive committee or audit committee.

"Many decisions and approvals were made informally and without evidence regarding who made those decisions or approved certain transactions," the report said.

Write to Matthew Cowley at matthew.cowley@wsj.com and Christopher Bjork at christopher.bjork@wsj.com

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