No-frills airline Ryanair has begun court proceedings against an Irish pilots' union in an effort to stop what it says is an organized campaign of harassment and intimidation being conducted through a web site.

"In addition to threatening and intimidating Ryanair pilots, this web site has also published specific threats of criminal activities which have been the source of a formal complaint to the Garda Siochana (Irish police)," Ryanair said in a statement on Wednesday.

The Irish Airline Pilots Associations (IALPA) and its British equivalent formed a group last year to represent pilots at Ryanair in an effort to secure greater trade union membership at the company.

"We reject any suggestion that any officials of IALPA engaged in the bullying, victimization or intimidation of anybody," IALPA President Evan Cullen said.

"We also point out that we invited Ryanair to participate in a third-party investigation into intimidation of any pilots within Ryanair. We understand that Ryanair has rejected those offers," he added.

The two pilots' associations say Europe's biggest low-cost airline is anti-union, while Ryanair says it recognizes employees' rights to join a union but most of its employees would prefer to negotiate directly with the company.