The chief executive of Aer Lingus, Mr Garry Cullen, resigned his £100,000-plus post suddenly yesterday, citing "personal reasons". The move was greeted with surprise in employee, trade union and political circles.

Mr Cullen said, given that privatisation was set to proceed - it is expected that the airline will float within 12 months - he had "given full consideration to the position of chief executive during the initial public offering (IPO) process and its aftermath and decided that, for personal reasons, this is a role that I'd not wish for myself".

Mr Cullen, who had been in the post just 16 months, said he believed it was necessary for him to clarify his intentions in advance of the IPO process.

It is understood he informed the chairman Mr Bernie Cahill, who then informed the Minister for Public Enterprise, Ms O'Rourke.

Mr Cahill said he accepted Mr Cullen's resignation with regret and praised "the major contribution" he had made to the airline's development during his 35-year career.

"Garry Cullen's contribution to Aer Lingus was central to the profitable growth of the business over recent years and, in particular, he played a major part in the strategic development of the airline," said Mr Cahill.

Mr Cahill insisted last night that Mr Cullen's resignation would not affect the flotation process.

Mr Cullen, who will miss out on a number of share options by not remaining with the company until after the flotation, had been the subject of some criticism by trade union leaders and analysts recently. They wondered if he was aggressive enough for the role.

Sources say it was a combination of issues, not just recent questioning of him being the right "fit" for the gruelling flotation process, which led to his surprise decision. They maintained that he had taken a brave decision - to go now rather than later if he felt he did want the role.

Mr Cullen (55), who is regarded as a very courteous man with a good knowledge of aviation, was not available for comment last night. A family member said he had gone out to dinner.

Ms O'Rourke said that Mr Cullen had been a great servant to the company and was part of the management team which had so successfully piloted its fortunes through difficult times. "It is a measure of the success of Mr Cullen's tenure, both as chief executive and in senior management, that the airline is now in a position to pursue a stock market flotation," she said.

"While it is regrettable to lose someone of Mr Cullen's talents, I fully understand his reasons for resigning at this time," she said.

Sources said that, unlike some of his predecessors, Mr Cullen enjoyed a good relationship with the Department of Public Enterprise. He was generally regarded as "competent and able".

Airline staff said they were saddened and very surprised at his departure.

One union source said last night that Mr Cullen had good "technical know-how", but perhaps lacked the pragmatism to draw the various strands together to prepare the company for flotation. "You need the capacity to be a clear leader," he said.

SIPTU represents about 90 per cent of the Aer Lingus employees. Last night, branch secretary Mr Tony Walsh said the union "accepts Mr Garry Cullen's decision, concurs with the sentiments expressed by the chairman and wishes Mr Cullen well in the future".