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.
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.