(Reuters) -A plan by JetBlue Airways ( JBLU ) to sign a domestic partnership with another airline faces opposition from its pilot union, which has warned the move could "fundamentally reshape" the U.S. carrier and impact the future of its workers.

The head of JetBlue's ( JBLU ) pilot union told members on Wednesday that a job protection provision in their existing contract does not allow the company to enter into a commercial agreement, according to a letter seen by Reuters.

On Tuesday, Reuters exclusively reported that the New York-based airline and United Airlines are negotiating a partnership, which is expected to focus on providing greater connectivity to customers and allowing them to earn and use frequent-flier miles.

In the letter, the JetBlue Air Line Pilots Association Master Executive Council Chair, Wayne Scales, said the union was working with attorneys and other professional staff to challenge threats posed by the proposed partnership.

"This is not a company operating with the best interests of its pilots in mind - at all," he wrote.

The union and JetBlue ( JBLU ) are currently negotiating a new contract for pilots.

JetBlue ( JBLU ), United and the pilots association did not immediately respond to requests for comment.

The airline has yet to return to sustained profitability after the COVID-19 pandemic. Slumping travel demand, driven by economic uncertainty linked to President Donald Trump's trade war, is worsening these financial challenges.

The airline has cut capacity due to weak demand in key markets, and offered early retirements to dozens of pilots to reduce labor costs.

One of the airline's few bright spots at the moment is the growth in its revenue generated from customer loyalty programs, aided by new partnerships. The company is relying on alliances with other airlines to bolster that revenue stream by offering customers greater connectivity.

Scales alleged that JetBlue ( JBLU ) had not consulted the union, or addressed its concerns about such an alliance. 

JetBlue's ( JBLU ) existing pilot contract permits the company to enter into a code-share agreement where one airline sells seats on flights operated by another airline, or into a joint venture only when its aircraft utilization and the number of active pilots on its payrolls go up.

JetBlue ( JBLU ) told investors on Tuesday that it was negotiating with a larger domestic airline without giving additional details, saying an announcement was expected in the current quarter. Industry sources told Reuters that JetBlue ( JBLU ) and United had yet to finalize the details and cautioned that the situation could still change.

JetBlue's ( JBLU ) aircraft utilization fell to 9.7 hours per day in the first quarter, down from 10.2 hours per day a year ago.

The company had about 4,800 active pilots in April compared with around 4,950 a year ago, according to a seniority list seen by Reuters.

"Until we have answers, every pilot at JetBlue ( JBLU ) has reason to be concerned about what's being negotiated behind closed doors," Scales wrote.