UPDATE 1-Ackman's Pershing Square exited ADP and United Tech investments -source

-source@ (Adds details on move)

BOSTON, Aug 5 (Reuters) - William Ackman's Pershing Square Capital Management has sold its positions in Automatic Data Processing Inc and United Technologies Corp and built a new position in a company whose name has not been disclosed, a source familiar with the matter said on Monday.

Ackman announced the exit from ADP in a letter sent to clients and seen by Reuters on Monday. Investors earned a 51% return, including dividends, on the investment and his firm Pershing Square made $1.2 billion on the bet which was made roughly two years ago, Ackman said.

Ackman has also told investors privately that the firm exited its investment in United Technologies Corp after the hedge fund manager in June urged the company to call off its planned merger with Raytheon.

Ackman sent a letter to the chief executive of United Technologies saying he was "extremely concerned" about the merger which is slated to occur after United Technologies spins off its Carrier and Otis businesses next year.

The announcements about ADP and UTX are the most significant moves Ackman has unveiled this year and come at a time his firm is making waves with double-digit returns as Ackman himself stays largely out of the limelight.

Through July, Pershing Square Holdings, Ackman's publicly traded portfolio, has gained 49.4%, compared with the average hedge fund's return of roughly 5% during the same period.

Ackman wrote that he exited ADP because he expects future returns to be more modest now that the market is "more accurately pricing in ADP's prospects for success."

He told analysts that the firm had exited United Technologies at a meeting last week.

A Pershing Square spokesman declined to comment on Monday. (Reporting by Svea Herbst-Bayliss in Boston Editing by Dan Grebler and Matthew Lewis)