United Airlines sued its pilots’ union on Wednesday, asking it to halt slowdowns that it said had led to the cancellation of hundreds of flights in the last 10 days.  

The lawsuit, filed in Federal District Court in Chicago, seeks a preliminary injunction against the Air Line Pilots Association and four pilots — Steven Tamkin, Robert Domaleski Jr., Xavier F. Fernandez and Anthony R. Freeman — claiming that the union and the pilots had organized an effort to encourage pilots to call in sick.

United said it was trying to avoid a repeat of a similar slowdown that occurred eight years ago, which led to what came to be known as the “summer from hell” for airlines.

The action by pilots in 2000 cost United hundreds of millions of dollars, and contributed to a financial slide that eventually landed the airline in bankruptcy protection.

United, the second-largest domestic airline behind American, said the campaign had been under way for a year but had accelerated in recent weeks. United said it had canceled 329 flights from July 19 to 27, disrupting plans for 36,000 travelers whose planes had no pilots to fly them.

United’s contract with its pilots’ union, reached while it was under Chapter 11 protection, expires in 2009.

Union officials have said they want to reopen the pact before then, saying they are working under a “draconian contract and work rules.”

Under the Railway Labor Act, airline union members cannot strike until their contracts expire and an impasse has been declared. A cooling-off period must then take place before a walkout can be called.

As a result, workers often resort to slowdowns or refuse to work overtime, prompting lawsuits by the airlines.