UPDATE 1-Italian union plans first-ever Ryanair pilot strike next week

(Updates with details and background)

DUBLIN, Dec 6 (Reuters) - Ryanair pilots in Italy plan to strike next week in what would be the first-ever such stoppage at Europe's busiest airline, a pilots union said on Wednesday.

The Irish airline is trying to recover from damaging cancellations caused by rostering problems, while pilots at less than a third of its 87 bases have accepted a pay offer.

The Italian union, which said the planned strike on Dec. 15 would last for four hours, is one of several across Europe that have been preparing for industrial action over demands for better conditions and an overhaul of Ryanair's collective bargaining system.

Ryanair does not recognize trade unions and a spokesman said it did not believe the pilots would follow through on the strike threat.

Italy's ANPAC, which said it represents 280 of Ryanair's approximately 4,000 pilots - and 40 percent of those based in Italy - said Ryanair pilots and cabin crew would not work between 1400 and 1800 GMT on Dec. 15.

Air traffic controllers and other airline workers will also strike on the same day, the union said.

The spokesman for Ryanair said ANPAC had "no role in Ryanair" and that the strike was the sixth that it and other Italian unions had announced before later being canceled.

"We expect this latest threatened strike will also be postponed canceled," the spokesman said.

Ryanair has never been subject to a pilot strike, although baggage handlers took industrial action briefly in 1998.

But pilots have mobilized in the wake of the announcement of 20,000 flight cancellations by the Irish carrier, which it blamed on a lack of standby pilots due to a failure in rostering following a rule change by Irish regulators.

Chief Executive Michael O'Leary told investors in September the airline had probably been "running too tight on pilot numbers" and the airline has since launched a hiring drive.

Ryanair routinely dismisses "competitor pilot unions" who it says claim to represent more Ryanair pilots than they do. But several unions in recent weeks have formed company councils and named serving Ryanair pilots as members.

Portugal's SPAC trade union this week announced that its members, which it says include a majority of Ryanair pilots based in the country, had voted to give it a mandate to call for industrial action up to and including a full strike.

Meanwhile, Ireland's IALPA trade union is due to ballot for industrial action this week.

O'Leary told pilots in September that there would be "consequences" for any pilots who take industrial action and that some aircraft could be taken out of bases if it were to occur. (Reporting by Conor Humphries; editing by Louise Heavens and Alexander Smith)