Cabin crew at Aer Lingus have threatened to ballot for industrial action in a dispute over crewing of new transatlantic services from Shannon commencing in January.

Eighty-six Aer Lingus cabin crew based at Shannon face uncertainty over their jobs in a row over outsourcing of more frequent flights using smaller Boeing 757 aircraft.

The airline accused the cabin crew union IMPACT of missing an opportunity for job creation by refusing to operate more frequent transatlantic flights from Shannon on aircraft leased from an external supplier.

Management has warned the union that as a result there will be a surplus of staff at the Shannon base.

It said the airline's next challenge will be to work through the "consequential issues" arising from this.

Sources said that could involve voluntary severance, or redeployment to Dublin or Cork.

In a letter to IMPACT Assistant General Secretary Michael Landers, Aer Lingus Director of Change and Engagement Sean Murphy described IMPACT's stance as a lost opportunity which would have provided an extra 40 jobs, and job security for the existing 86 staff.

In July, Aer Lingus announced that it was almost doubling the frequency of its transatlantic services from Shannon from January 2014, using smaller Boeing 757 aircraft.

Aer Lingus said its original plan was that the aircraft will be leased from external provider ASL Aviation, but would be operated with a crew of four Aer Lingus cabin crew staff per flight.

He notes that the approval of the Irish Aviation Authority had already been secured for this operation - including the requisite crewing numbers.

However, the union has written to the airline saying it will not cooperate with the crewing arrangement.

In response, Aer Lingus management said it is clear that IMPACT will not be cooperating with the commencement of the 757 operations from January, for which tickets have already been sold.

As agreement has not been reached, Aer Lingus has today informed ASL Aviation that Aer Lingus cabin crew will not resource the operation, and that the required staff should be sourced elsewhere.

IMPACT Communications Director Bernard Harbor warned that if Aer Lingus moved to outsource crewing arrangements, the cabin crew will ballot for industrial action.

He said there had been no discussion regarding whether there should be four or five cabin crew per flight.

He said Aer Lingus had insisted on four crew with no discussion on a "take it or leave it" basis - and that Aer LIngus had refused to supply information requested by the union to enable discussions.

Aer Lingus pilots have already agreed to operate the new services as part of a broader complex agreement.