Activist investor Bill Ackman is urging United Technologies to abandon its $100 billion stock merger with Raytheon, a source familiar with the matter confirmed to CNBC.

Ackman, whose Pershing Square Capital Management owned 5.8 million United Technologies shares at the end of the first quarter, said in a letter that the tie-up is ill-advised. Ackman had previously encouraged United Technologies to streamline its business by splitting up parts of the conglomerate.

The fund manager sent a letter to United CEO Greg Hayes on Sunday morning following a report that the company was in merger talks with Raytheon, the source confirmed to CNBC. Pershing Square declined to comment on the letter.

In a statement to CNBC, United Technologies said, "We are confident that our shareholders will see the merits of this transaction and the value it brings to them and the company. We will be working diligently in the days and weeks ahead to make sure that the details of the transaction are presented to and fully understood by all shareholders."