Private companies in Europe are facing greater pressure from bondholders to make their earnings public, according to an upcoming report by investor lobby group European Leveraged Finance Alliance (ELFA).

About half of those surveyed by ELFA said they've avoided investing in companies that request passwords, while 85 percent said they take a negative view of such debt issuers. About 38 percent of private high-yield borrowers restrict investor access to their financial data in some way, according to a separate search by Bloomberg.

Many privately owned companies keep their earnings behind password-protected websites, with some frequently changing their access codes. Others limit investor access by requiring proof of bond holdings or scrub financial data from their sites after redeeming their debt. Some companies have started requiring investors to keep information confidential with lengthy contracts akin to non-disclosure agreements (NDAs), according to ELFA's report.