The Conference of Peripheral Maritime Regions (CPMR) has called for the importance of regions to be fully heard in the upcoming Brexit talks between the EU and UK.
Speaking on Tuesday, CPMR president Vasco Cordeiro cited Brittany and Normandy as two examples of how Brexit will impact on Europe's regions.
The UK is the number one market for tourism and also for second home ownership in Normandy, he said.
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Britain is also the third largest export market for the northern French province. Similar figures apply to Brittany.
Another example, he said, is the Basque region in Spain - the UK is its fourth biggest export market.
"These regions rely on Britain for a range of things and there is no doubt that the biggest impact of Brexit for Europe's regions will be economic. This is clear and that is why the voice of regional authorities needs to be taken fully into account in the Brexit negotiations."
He added, "Unfortunately, from what I have seen so far there is going to be room for improvement on this."
Cordeiro said that events such as Brexit were a consequence of growing populism in Europe and a growing public disenchantment with the EU and the political elite.
Some of Europe's maritime and outermost regions, he said, were the first to feel the impact of challenges such as migration and climate change.
The CPMR, which represents 160 regions in 25 European countries, has issued a political statement setting out its proposals for EU reform which, is says, is vital to re-energise the European project.
The organisation says the success of Emmanuel Macron in the French presidential elections at the weekend represents a positive turn of events.
In a briefing with Brussels-based reporters, Cordeiro said, "In the French election, we saw a populist approach being faced, contested and beaten. However, Europe's challenges have not gone away."
Cordeiro, who is based in the Azores, "We hope the election of Macron means our reflections on Europe can focus on driving forward a positive reform agenda where regions are integrally involved.
"The risk, if this doesn't happen, is that disillusionment with the EU project will resurface as new challenges arise."
EU reform, he said, does not necessarily mean a two speed Europe but, rather, should take into account the role regions have to play.
"The only way we are going to have a European project is for all parties to play a part in it. This is the challenge."
The CPMR, which is based in Rennes, France, says cohesion policy, which has been questioned in some quarters, should be a fundamental pillar of the EU.
CPMR secretary general Eleni Marianou said, "The EU budget must prioritise support for cooperation activities between regions in the future with a much greater focus on actions that address the challenges and concerns of European citizens."