(Adds details on process, background paragraphs 5-9)

FRANKFURT, May 25 (Reuters) - Lufthansa's aircraft maintenance business Lufthansa Technik will enter the second phase of its partial sale process in mid-June, two sources familiar with the matter told Reuters.

The bidder field has narrowed to include private equity firms Advent International, CVC Capital Partners and Bain Capital, all vying to invest in a minority stake, the sources said, speaking on condition of anonymity.

Blackstone had previously shown interest in the asset but is no longer involved, the sources said.

Lufthansa Technik, Advent, CVC, Bain and Blackstone declined to comment.

The second round of the process will grant bidders access to data to carry out due diligence, the sources said.

Lufthansa hopes for proceeds of up to 1.5 billion euros ($1.65 billion) for the sale of a 20% share in its maintenance arm, though sources have cautioned a deal may not happen if the parent company does not receive high enough offers.

Lufthansa Technik CFO William Willms previously said there would be two rounds of negotiation by the summer.

The airline is seeking a long-term investor and has said it would consider a partial stock market listing of Technik at a later point in time.

Lufthansa and the Italian Treasury on Thursday reached a deal for the German airline to take over a minority stake in ITA Airways, the successor of Italy's former national carrier Alitalia. ($1 = 0.9084 euros) (Reporting by Emma-Victoria Farr and Ilona Wissenbach in Frankfurt, editing by Kirsten Donovan)