Raytheon Co. (RTN) - Get Report and United Technologies Corp. (UTX) - Get Report shares were active Wednesday after one of Wall Street's most high-profile activist investors is reportedly ready to oppose their planned $120 billion merger.

William Ackman, who runs the activist hedge fund Pershing Square Capital Management LP, contacted United Technologies' CEO Greg Hayes shortly after the deal was made public Sunday, according to sources speaking to the Reuters news agency and providing them with the correspondence. Multiple media outlets have also reported the Ackman email. Ackman, whose fund owns around 0.67% of United Technologies, had previously supported the group's plans to split into three separate businesses.

"We are extremely concerned that such a transaction will significantly lower the business quality of pro-forma United Technologies' aerospace business, and, to make matters worse, will be accomplished through the highly dilutive issuance of large amounts of United Technologies stock," the Ackman read.

United Technologies shares were marked 1.3% higher at $124.57 Wednesday, while Raytheon was seen 1.65% lower at $174.73. Both shares are lower than their Friday closing prices, the last day of trading before the deal was announced on Sunday.

"We are confident that our shareholders will see the merits of this transaction and the value it brings to them and the company," United Technologies said in a statement sent to multiple media outlets. "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."

Ackman's objection follows concerns for the deal, which would be the largest-ever aerospace and defense sector tie-up, expressed earlier this week by President Donald Trump, who told CNBC that he was concerned the tie-up could reduce competition in the defense sector and give the Pentagon reduced leverage in negotiating future procurement contracts.

"I want to see that we don't hurt our competition," Trump said Monday . " I hope (the deal) can happen. But I don't want to see where we have one less person that can compete for an order. I don't want to see that. It's no good."

Raytheon Technologies Corp., the proposed name for the newly-created group, with have combined sales in the region of $74 billion, the companies said in a statement published Sunday, and will be listed on the New York Stock Exchange.

Raytheon, which makes the Tomahawk and Patriot missile systems for the U.S. military, will own 43% of the combined group, which will be lead by United Technologies CEO Grey Hayes. Raytheon's Tom Kennedy will serve as executive chairman for a minimum of two years.