Ryanair's plans to expand services from Belfast International Airport have stalled because the airport will not risk alienating its existing low-cost airline clients, senior industry sources have said.

The airline is one of several low-cost carriers that have talked to airport owner TBI about taking over available slots following the decision by British Airways and BMI British Midland to axe key routes from the airport.

Industry sources say the prospect of Ryanair introducing a new Northern operation is increasingly unlikely because it cannot agree terms with TBI.

Ryanair is believed to have sought costings for introducing 12 flights a day from the airport, chiefly to destinations in Britain. The airline operates a service out of the City of Derry airport to London Stansted that last year carried more than 125,000 passengers.

However, it is understood that discussions between Belfast International and Ryanair have not proved fruitful and it is possible that Ryanair may look at extending its Derry services.

Industry sources say Ryanair is demanding terms that TBI cannot fulfil without seriously undermining its existing relationship with easyJet and Go, which are also in discussions about extending their services from the airport.

Easyjet operates daily flights to four UK cities and Amsterdam, and is in talks to increase frequency and expand its international routes from Belfast.

Go has doubled the frequency of its London services and may look to increase the number of UK cities it serves from the North.

It is understood Ryanair wants to set up a number of services that would compete with these existing low-cost operators and is seeking a "favourable" introductory offer to come to Belfast International.