Summer flight schedules show that US flights to Shannon are falling in the run up to the beginning Open Skies policy which will see the end of the stopover at the Co Clare airport.
The Irish Tourist Industry Confederation (ITIC) published a report today showing that Western tourism will suffer from this winter onwards.
ITIC chairman Dick Bourke said that while Open Skies will ultimately benefit tourism, a plan to arrest the short-term negative effect should be implemented urgently.
The ITIC report says preliminary schedules for the coming winter show an increase from 32 to 76 of weekly trans-Atlantic direct flights to Dublin, with non-stop flights to Shannon drop from 48 to 27.
Current schedules for the Summer of 2008 show a weekly increase from 77 to 97 direct flights into Dublin, with Shannon's current 2008 schedule showing a drop from 56 to 42 weekly services.
Mr Bourke, said: "We know for certain that Ireland will have service from at least 3 additional US gateways next year, and that is a great start in the first year of the liberalised era. But we must build on that and seek more new services over the coming years.
"The Government must act immediately to implement a tourism and economic plan for the Western regions which will be negatively impacted in the short term by the reorientation of air services between the US and Ireland.
We cannot lose any time as the impact will begin to be felt this winter."
Open Skies is an international agreement to liberalise the aviation market and end restrictive practices such as the requirement that some licenses for trans-Atlanctic routes must stop at Shannon even if they preferred destination is Dublin.
Research show US holiday visitors who fly directly to Dublin spend slightly less time in Ireland and visit fewer Western regions than those who arrive directly in Shannon, the ITIC says.
The report concluded that a continuation of the trend could lead to a net loss of between 150,000 and 190,000 bednights in the State.
It also says there will up to 25,000 fewer Americans staying at least one night in counties Cork and Kerry, and up to 40,000 fewer visiting the Shannon region. The Galway West region could see a drop of up to 15,000 US holiday visitors.
This would also lead to a redistribution of tourism services, such as car rental and coaches services, from Shannon to Dublin.
The report recommends: a three-year budget for Tourism Ireland to attract US holiday-makers to the West; a development plan for expanding trans-Atlantic air services; the development of air services from the UK and mainland Europe to the West.
Mr Bourke also welcomed the recent decision by the European Commission to block Ryanair's proposed takeover of Aer Lingus. He said had the takeover gone ahead, the a large private monopoly would in effect have replaced two active competitors.