* Mitsubishi Aircraft makes claim in countersuit
* Bombardier says M'bishi Aircraft illegally stole trade secrets
* Mitsubishi Regional Jet would compete with Bombardier's CRJ (Adds Bombardier statement, dateline)
MONTREAL, Jan 28 (Reuters) - Mitsubishi Aircraft Corp on Monday accused Bombardier Inc of using "anticompetitive conduct" to limit competition for planes with under 100 seats, as the Japanese company works to bring its long-delayed regional jet to market.
In October, the Montreal-based plane-and-train-maker sued the aircraft unit of Mitsubishi Heavy Industries Ltd, alleging that former Bombardier employees passed on trade secrets to help with the development and certification of the company's new MRJ regional jet.
The MRJ would compete with regional jets produced by Bombardier and Brazil's Embraer SA.
Mitsubishi said in a counter-filing claim that Bombardier tried to coerce it and Seattle-based Aerospace Testing Engineering & Certification (AeroTEC) to sign no-poach agreements in 2016 that would stop them from hiring Bombardier's employees.
Bombardier Chief Executive Alain Bellemare also implicitly threatened the continuation of its "supply relationship" with Mitsubishi Heavy in a 2016 letter to board chairman Hideaki Omiya unless the Japanese company "ceased the solicitation of Bombardier employees," Mitsubishi said in the claim.
"We believe that in response to the threat posed by the MRJ, Bombardier has chosen to engage in a pattern of anticompetitive conduct instead of competing on the merits of its product," said Mitsubishi in a company statement.
"Bombardier has threatened, pressured and sought to coerce Mitsubishi Aircraft, its U.S.-based partners, and individual employees working on the MRJ program."
Mitsubishi's regional jet program, Japan's first passenger plane since the 1960s, has been delayed by several years, with first customer ANA Holdings Inc now expecting the 90-seater plane in 2020, rather than in 2013 as originally envisaged.
Bombardier has said it hopes to make a decision this year on the future of its money-losing CRJ regional jet as it "aggressively" pursues new orders to secure its industrial future.
In response to the latest filing, Bombardier repeated its view that its rival was illegally trying to obtain and use its trade secrets.
"Mitsubishi Aircraft Corporation's attempt to recast the dispute as anything other than Bombardier's justified protection of its intellectual property is misguided and disingenuous," it said in a statement.
"Bombardier will vigorously enforce its rights and that includes holding (Mitsubishi Aircraft), AeroTEC and individual wrongdoers accountable."
In the October lawsuit, Bombardier accused Mitsubishi Aircraft of violating the Defend Trade Secrets Act of 2016 by trying to use the confidential data and documents obtained from former employees to accelerate the "extremely complex and costly" process of getting its planes certified.
Mitsubishi Aircraft asked a U.S. court in December to dismiss Bombardier's lawsuit, arguing that the allegations were baseless and designed to "disrupt development" of its rival jet.
Reporting by Allison Lampert in MONTREAL; Editing by Christopher Cushing