"We're not talking about leasing slots or planning to change anything at Heathrow, " said a spokesman for the company.

He confirmed, however, that the company entered into a short-term lease agreement "for a couple of years" covering one of its slots with Continental earlier this year. It also has another longer-term lease deal with British Airways.

He said that the airline felt that those slots had no meaningful value due to the times they covered. Aer Lingus is the fourth-largest holder of slots at the airport.

Continental is currently seeking slots at the airport so it can provide long haul services from Newark in New Jersey and Houston, Texas to Heathrow after the introduction of Open Skies next March. A spokesman for the airline refused to comment on whether it had entered into talks with Aer Lingus.

Although Aer Lingus is effectively barred from selling slots without the government's consent, it can enter into short-term leases of up to three years on its Heathrow slots.

However, if a slot is leased for more than three years in any four-year period, the slot sale regulations imposed by the government on the airline are automatically brought into operation.

Although Heathrow slots have become particularly valuable in recent months as airlines gear up for Open Skies, aviation analyst Neil Glynn of NCB Stockbrokers, said it could prove difficult for Aer Lingus to unlock their value through rental arrangements.

"There is a grey market for slots but the timing of the slots is crucial in determining their value. Aer Lingus is a short haul operation so its slots may not be suitable for the longhaul carriers who are seeking them."