Unions at Aer Lingus are warning of possible flight disruption for passengers next week following a new threat by the airline's chief executive Dermot Mannion.
In a letter to pilots, Mr Mannion threatened to suspend all pilots on Tuesday morning unless they agree to co-operate with the new Belfast base.
Pilots have described the threat as madness and an attempt to shut the airline.
A fifth pilot was suspended today, following the suspensions of four pilots yesterday after they refused to take part in training of new recruits for the company's Belfast base.
The Labour Relations Commission has been in contact with both sides and is assessing the situation, but there is no immediate prospect of talks.
Management sources have said it has already been to the Labour Relations Commission and IMPACT says today's letter by Mr Mannion is aimed at provoking industrial action.
The union says it is open to talks but is refusing to co-operate on the new Belfast base.
In the absence of negotiations, the dispute is likely to get worse before it gets better.
