(Adds details of BNDES statement, merger context, analyst comment)
SAO PAULO, July 8 (Reuters) - Brazil's state development bank BNDES lashed out at Portugal Telecom SGPS SA over a debt deal souring relations with Grupo Oi SA, highlighting the mounting pressure threatening their planned tie-up.
BNDES said on Tuesday that recent bond investments by Portugal Telecom are "inconsistent with minimum standards of good corporate governance." The bank said it is pressing for more information as it aims to protect the interests of Oi shareholders, including itself.
Oi said last week that Portugal Telecom did not make a timely disclosure of its investment in Rioforte, which is held by a key shareholder under investigation in Luxembourg.
Preferred shares of Oi tumbled nearly 6 percent, extending a 17 percent loss over the past month. Portugal Telecom dropped 5.1 percent in Tuesday trading.
Portugal Telecom has said it was intent on resolving questions over the 897 million euros ($1.22 billion) in debt from holding company Rioforte, owned by the Espirito Santo banking family, a major shareholder in the Portuguese company.
The debt, equal to roughly 40 percent of Portugal Telecom's market value, matures next week, and was hanging over the Oi merger formalized in an April 28 share offering.
The merger was touted as a chance to strengthen Oi's corporate governance after years of bickering between minority and controlling shareholders, but distrust of the Portuguese partner is clouding those hopes.
"We believe transactions with large shareholders without proper disclosure to minorities are negative to corporate governance," wrote JPMorgan Securities analyst Andre Baggio, who cut his rating on Oi shares to "underweight" on Monday.
Baggio added that if Oi pushes for a lower valuation of Portugal Telecom's assets in their merger, the improved terms for Oi should boost its share price.
BNDES reiterated its support for Oi's current management, including Zeinal Bava, the chief executive officer of both Oi and Portugal Telecom's Portugal unit since June 2013.
BNDES is the main source of long-term lending in Brazil and a major shareholder in several of the country's so-called "national champions." The bank was a key sponsor of the 2008 merger that created Oi as well as the current tie-up with Portugal Telecom. (Reporting by Guillermo Parra-Bernal and Brad Haynes; Editing by Jeffrey Benkoe)