MADRID, June 30 (Reuters) - American buyout fund Apollo on Friday filed a tender offer for 100% of the shares of Spanish industrial testing company Applus Services in a transaction that values the company at 1.23 billion euros ($1.33 billion).

The fund offered to pay 9.50 euros in cash per Applus share, it said in a filing to the stock market regulator.

The regulator had suspended trading in Applus shares early on Friday, and authorised it to resume at 1 p.m. (1100 GMT).

Apollo's bid offers a 1.9% premium over Thursday's closing price of 9.32 euros. The shares have gained 45% so far this year on speculation several investors were preparing a takeover.

Reuters reported last month that several funds including Apollo, Apax, I Squared Capital Advisors and TDR were looking at Applus.

Last week, infrastructure private equity fund I Squared Capital Advisors said it was evaluating a takeover bid for Applus together with TDR Capital.

($1 = 0.9223 euros) (Reporting by Inti Landauro and Jesus Aguado, editing by Andrei Khalip)