Commercial director Enda Corneille confirmed that management now believed "people purposely changed their shifts around so they would be there at 6am when the action occurred".

The strike, which saw around 100 staff stop work for an hour to attend a protest meeting, caused delays of up to 90 minutes for thousands of passengers who were scheduled to travel on early flights.

Corneille refused to comment on whether the airline believed Siptu had orchestrated the action. However, it is understood the investigation will conclude that neither the union's Aer Lingus branch nor its national leadership was involved.

The investigation, which concluded last Friday, is believed to have focused on a small number of Siptu shop stewards whom management alleges were behind the action.

It is understood management will decide on the fate of the workers at the centre of the investigation this week.

The walkout is believed to have centred on the airline's attempts to introduce more flexible working arrangements, particularly during peak periods.

According to Corneille, the airline used to employ many temporary workers every summer, "whereas now what we want to do is, rather than have a huge turnover of seasonal staff, we want to make the staff more productive". He said the airline wanted to achieve this through multiskilling, with staff expected to fill several different roles.

Siptu's national industrial secretary, Michael Halpenny, said it was opposed to the airline's plans, which he said had already led to severe understaffing at Dublin airport. He said the walkout was a spontaneous reflection of union members' frustration with this.

"There is an acknowledged agreement that allows flexibility on some shifts which dates back to last September but the company is trying to go beyond that, " he said.

Halpenny refused to comment on the possibility of more unofficial action at the airport or what his union's response would be if Aer Lingus took action against workers as a result of its investigation.

"There is no point in speculating, " he said. He said the union's main focus was on responding to the company's decision, following the action, to impose a deadline of 1 August on agreement for its cost-cutting plans.