Apollo had made a cash bid for Applus at 9.50 euros per share in June.

Apollo's agreement with 15 investors in Applus is the latest episode of a bidding war pitching the U.S. fund against infrastructure fund I Squared Capital Advisors and TDR Capital, who offered 9.75 euros per share.

Spain's government last month authorised Apollo to take over Applus, while the stock market regulator is still assessing I Squared and TDR's offer.

Following Apollo's move, its rivals now have a chance to increase their offer price.

Applus shares traded 5% up on Wednesday morning at 10.84 euros, above the last price offered by Apollo.

Through its share purchase agreement, Apollo is gaining a hedge over its rivals and complicates a potential higher bid, Renta 4 analysts said in a report.

TDR declined to comment. I Squared was not immediately available for comment.

(Reporting by Matteo Allievi, editing by Inti Landauro and Jason Neely)