MADRID, April 25 (Reuters) - Saudi Arabia's STC Group and sovereign fund PIF may bid for the Austrian unit of Spanish mobile phone tower operator Cellnex, El Economista newspaper reported on Wednesday evening citing unidentified industry sources.

The unit is worth at least 800 million euros ($857 million), the newspaper added.

STC built a stake of 4.9% in Telefonica shares and financial instruments that give it another 5% in what it called "economic exposure" to the company.

STC has still not requested government authorisation to exercise the voting rights corresponding to the financial instruments.

Spokespeople at Saudi Telecom did not immediately respond to requests for comment. Cellnex declined to comment.

($1 = 0.9332 euros) (Reporting by Inti Landauro; editing by Jason Neely)