Ryanair will have to repay €4.8 million to Belgium's Charleroi Airport if the European Commission rules that the subsidies and incentives it received there were illegal.

In information filed with the US Securities and Exchange Commission, the Irish low-cost airline said that adverse rulings in the Charleroi investigation or in others could result in the scaling-back of its growth.

"Ryanair's future growth is materially dependent on its ability to access suitable airports located in its targeted geographic markets at costs that are consistent with Ryanair's low-fares strategy," the company says.

In the document, Ryanair states that the full amount of discounts it received at Charleroi would amount to €2.6 million. In addition it received a further €2.2 million in marketing and other supports.

The document cautions that a change in the terms of Ryanair's access to these facilities or any increase in relevant charges it has to pay could have "a material adverse effect on the company's financial condition and results of operations".

The airline also states that it could face significant additional costs arising from legal proceedings regarding Charleroi and Strasbourg.

Ryanair has already suspended its services to Strasbourg after a French court ruled that the money it received there constituted illegal aid.

The company has appealed this decision and has said it will re-open the route if the appeal is successful.

Ryanair senior management have been briefing analysts about the impact of the Commission's Charleroi investigation on the airline.

Sources suggest the airline has been playing down the financial impact and continue to express confidence about the outcome of the inquiry and the likely terms of any future agreements at state-owned airports.

As well as repaying any money, the company could be forced to renegotiate the deals with other publicly-owned airports.