Aer Lingus staff may strike next month in opposition to proposed changes in working practices at the airline.

Both Siptu and Impact, the two biggest unions at the airline, are balloting their members for industrial action, which if successful could result in a strike as early as the start of next month.

The unions, which represent a range of Aer Lingus employees including pilots, cabin crew and general staff, are objecting to proposed changes in working practices, such as holiday and overtime pay and grading structures.

These proposed changes were outlined to the unions in December as part of a cost-cutting exercise entitled "Programme for Continuous Improvement". On Monday Aer Lingus chief executive Dermot Mannion sent a letter to the unions saying that the company was pressing ahead with the changes, starting on February 1st for new employees and the start of March for existing employees.

In the letter, Mr Mannion said Aer Lingus continued to be out of line with its competitors in terms of its cost base and given its strong growth and expansion plans, this situation was no longer sustainable.

In the original document issued in December, Aer Lingus revealed that its payroll costs - at an average of more than €72,000 per worker - had risen by almost 30 per cent in the past three years and as a result were higher than most other major airlines in Europe.

It also said the policy of staff receiving two lieu days for every public holiday would have to be abolished and that all employees would have to work 37.5 hours per week.

However, the unions are objecting to the upcoming deadline for implementing the changes, accusing the company of breaking promises made prior to its flotation by forcing changes without consultation.

In a letter sent to Mr Mannion on Friday, Siptu's national industrial secretary, Michael Halpenny, said that by proceeding with the changes, the company was in "flagrant breach" of commitments entered into in August under the revised 2004 business plan.

Following a request by the unions, all parties involved in the dispute are due to meet at the Labour Relations Commission on Friday to discuss the issue.