Boeing Co. and Airbus SE voiced concerns about a proposed tie-up of two leading suppliers today, potentially upsetting United Technologies Corp.’s $23 billion acquisition of Rockwell Collins Inc.

The world’s largest planemakers added to the chorus of skeptics to a deal that creates an aerospace behemoth with a range of products to outfit jetliners and warplanes. Investors sent United Technologies shares plunging by the most in two years, while credit ratings companies said the manufacturer risks downgrades. Boeing, which has been squeezing suppliers for discounts, warned it would take action to protect itself if it saw the merger as harming its business.

“Until we receive more details, we are skeptical that it would be in the best interest of — or add value to — our customers and industry,” Boeing said in an emailed statement. “Should we determine that this deal is inconsistent with those interests, we would intend to exercise our contractual rights and pursue the appropriate regulatory options to protect our interests.”

Planemakers are wary of distractions the merger could create for a key supplier just as the planemakers embark on the biggest production ramp-up in history for single-aisle jets, their largest source of profit. The merged aerospace businesses, to become a separate unit called Collins Aerospace Systems, will make seats, landing gear, flight controls and other systems, and the data pipelines linking pilots and passengers to the Internet.

Boeing and Airbus will have a say on the merger’s fate because they both have “disproportionate influence” on deals throughout their supply chains, Nicholas Heymann, an analyst at William Blair & Co., said before the deal was announced. The planemakers hold contractual clauses that give them broad authority over parts production, essentially making each customer “a gatekeeper for potential structural changes in the supplier base,” he said.

Airbus, one of United Technologies’ major customers, pressed the company to make sure it can keep up with commitments to deliver its Pratt & Whitney jet engines on time after a rocky rollout for the company’s geared turbofan.

“We hope that this M&A would not distract UTC from their top operational priority,” an Airbus spokesman said in an emailed response to questions after United Technologies confirmed the cash and stock deal for Rockwell Collins. “Our total focus is on delivering planes.”

United Technologies’ Pratt unit builds engines for Airbus’s best-selling A320neo series, but its new geared turbofan technology — which cost $10 billion to develop — has been beset by manufacturing hurdles, delivery delays and technical glitches. That’s elicited stark rebukes from Airbus.

United Technologies dropped as much as 5.4 percent to $111.57 in New York today, the biggest intraday drop since August 2015. The company plans to hold calls today and Sept. 6 with customers to discuss the merits of the deal, Chief Executive Officer Greg Hayes said in a phone interview.

“We didn’t want to get ahead of ourselves, the board literally voted yesterday to do this deal,” he said. “We’ll be talking to all of them, trying to make sure they understand the benefits of bringing these companies together.”

Pratt’s engines are one of two options for A320neo models, competing with products from the CFM International joint venture of General Electric Co. and Paris-based Safran SA. While Pratt has signed just one buyer this year, the CFM turbine has snatched up about 10 times more orders amid the United Technologies unit’s operational challenges.

Airbus CEO Tom Enders said in July that the revamped A320neo narrow-body workhorse is a major concern, with the company hitching its earnings goals to Pratt meeting delivery targets and providing a reliable fix to problems.

“There are just too many maturity issues on this engine,” Enders said at the time. “That is frustrating for us, that’s frustrating for the customers.”