NEW YORK, March 19 (Reuters) - Shares of Nasdaq fell 3% in extended trading on Tuesday following the announcement of a secondary share offering that will allow Borse Dubai to slash its minority stake in the U.S. stock exchange.

Borse Dubai had been the largest single shareholder in Nasdaq. It said it will reduce its ownership stake to 10.8% from 15.5%.

Borse Dubai will offload about 26.96 million shares of the exchange operator as part of a secondary offering, according to a statement released after the closing bell on Tuesday.

Nasdaq shares closed the regular session up 3.6% at $62.46. At that price, Borse Dubai would receive about $1.7 billion from the share sale.

"Subject to completion of the offering, Borse Dubai intends to agree to an 18-month lock-up of its remaining shares controlled by Nasdaq, with a right to designate a nominee to the Nasdaq board of directors so long as it continues to own at least 10% of Nasdaq's outstanding shares as of the date hereof," the statement said.

Following the offering, Borse Dubai will become the second-largest shareholder in Nasdaq, with about 62.4 million shares or about 10.8% of the company's outstanding shares.

U.S. private equity firm Thoma Bravo will become Nasdaq's largest shareholder, holding a 12.5% stake in the U.S. exchange with about 71.6 million shares. (Reporting by Chibuike Oguh in New York Editing by Matthew Lewis)