With the important referendum vote on Great Britain's membership of the European Union almost upon us, the country's airports and airlines are urging a vote to remain or face increased costs and a downturn in travel.

A key voice in the debate is London Heathrow Airport — the United Kingdom's only large hub, and its busiest gateway.

"Heathrow believes that the U.K. will be better off remaining in a reformed EU," Heathrow CEO John Holland-Kaye said in a statement. "We are the UK's only hub airport, connecting Britain to over 80 long-haul destinations, and handling over a quarter of U.K. exports — but we recognize that for business to thrive we also need to be part of the single European market."

Being a member of the EU gives airlines the right to operate point-to-point services to, from or within any member country. This has seen the likes of Ryanair, Flybe and easyJet take advantage of the ability to open routes across the continent regardless of the location of their headquarters.

Leaving the EU would mean British airlines no longer automatically had this right without potentially expensive negotiations and granting of rights. Alternatively, airlines would be forced to set up subsidiary companies within each country they operated in order to continue with routes that do not originate in the U.K.

Large airports such as Heathrow, Gatwick and Manchester believe their place among the best in Europe could be put at risk if airlines are forced to put up prices or cut flights from their schedules as a result of an exit.

Those in favor of a "Brexit" believe, however, leaving the EU would strengthen the case for expanding Heathrow and a growth in demand for flights as new global links were sought with the country's new trading partners.

One particular market where growth could be seen would be with direct flights to China — a huge emerging market for the West. Airports such as Birmingham and Manchester have already seen new direct links to China, and Heathrow could see a surge in this market should an exit occur.

Following a letter signed by more than 200 businesses, including Heathrow, Gatwick and Manchester Airports Group (MAG), Charlie Cornish, chief executive of MAG, said: "The EU has benefited U.K. consumers and businesses enormously over the last 25 years by opening up the European aviation market to provide greater competition and choice.

"U.K. airlines now compete successfully on routes right across Europe, taking full advantage of the 'open skies' created by the EU."

While a leave vote may stimulate growth in new areas and lead to investment in years to come, it may in the short term cause disruption for travelers and businesses that rely on the network of flights, and pressure for airports to provide connections to new markets.

The decision on whether to expand either Gatwick or Heathrow airports with an additional runway has been delayed until after the EU referendum so that the environmental and economic impact of the decision can be further assessed.