The Government is set to examine whether valuable landing slots at Heathrow Airport owned by Aer Lingus should form part of the sale of the airline.

A discussion paper from the Minister for Transport, Mr Brennan, will examine whether the slots should be retained by the State because of their national strategic value or be included in the sale.

The question of the slots is likely to preoccupy the airline and the Government in the next few months as plans to sell the airline intensify.

Mr Brennan intends to publish the discussion paper and facilitate a Dáil debate where the slots issue will be discussed.

Opposition politicians are pressing the Minister to find some way to retain ownership or control of the slots, which are believed to be worth up to €120 million.

They believe at least some of the slots should be retained in State ownership because they are national assets.

In October, British Airways paid £12 million (€17 million) for four slots from US carrier United Airlines. Aer Lingus has 20-30 slots, depending on the time of the year.

It is understood that Aer Lingus opposes withholding slots from any sale process because the airline's value would be seriously eroded.

Opposition politicians, however, are concerned that a buyer might buy the slots for use on non-Irish routes.

Some Opposition politicians have raised the possibility of British Airways buying Aer Lingus and using the slots at Heathrow to increase its services into North America.

During a debate before Christmas, Mr Brennan said on the issue: "I share the concerns of the House. We must be careful as to how we handle the issue because it is essential that we continue to have access to these hubs in the future.

"This whole area of slots is a legal grey area, which is exercising EU policymakers' minds. I share the deputies' concerns on this matter and will make stringent efforts to ensure that, in any changes of Aer Lingus ownership, great care will be taken on that issue."

But he made it clear that the State did not own the slots per se.

"I must clarify that Ireland does not own slots in Heathrow Airport.

"These slots are time slots held by Aer Lingus on the basis of a historical precedent, known as 'grandfather's rights'.

"The allocation of slots at congested airports such as Heathrow is strictly governed by EU rules which are now under review. The current regulation allows for the exchange of slots but does not cover their sale. This has not prevented the development of a 'grey market' in slots," he said in the Dáil.

Analysts have put a price of €350-€500 million on Aer Lingus.

If the slots were in some way excluded from the sale, these valuations might have to come down.

In the recent debate, Mr Brennan explained why the slots were important strategically.

"The slots at Heathrow are important because, apart from Aer Lingus, only British Midland operates Heathrow-Ireland services. Their value arises from their scarcity." While increased runway capacity at Heathrow may provide additional slots in the years ahead, as the largest international hub in the world the slots will remain highly valuable.

Fine Gael has described the slots as "extremely valuable" and pointed out that "if Aer Lingus were to lose them we might not be able to get them back.".

Mr Simon Coveney, of Fine Gael, speaking in the debate in the Dáil, said: "It is crucial that the number of slots held by Aer Lingus at Heathrow for flights to and from Dublin, Cork and Shannon be maintained.

"It is important, therefore, that we should not lose what I consider to be a national asset, namely, our slots at Heathrow through the sale of Aer Lingus."