France and Germany are among 20 European nations that restrict people with mental health needs from taking part in polling.
With the future of the European Union at a crossroads, three quarters of member states plan to prevent or restrict people with mental health needs from having a say in how it progresses.
European Parliament elections will take place this time next year, just weeks after Britain's planned exit begins.
It remains to be seen how international voting preferences will be influenced by Brexit.
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However, in three quarters of EU member states, individuals living with a mental illness stand to be shut out of participating.
Hundreds of thousands of people in Europe with mental health problems live under 'full guardianship' – a form of substitute decision-making where a person is deprived by law of their legal capacity and, in some instances, their right to vote.
Research by Mental Health Europe, published earlier this year in the report “Mapping and Understanding Exclusion in Europe”, found that at least 264,000 people live under full guardianship, but the limited availability of data means the true figure could be 500,000 or higher.
Nine EU countries wholly exclude people with mental illness from participating in elections:
Eleven EU countries place restrictions on people with mental illness from voting, through varying 'case-by-case' processes:
- Czech Republic
Just eight existing EU members place no mental health criteria restrictions and support full participation:
- United Kingdom
“Everyone should have the right to an equal say about how Europe is run," said Maria Nyman, Director of the Mental Health Europe NGO.
"States need to reform outdated laws and support people with psychosocial disabilities to make decisions about their lives, including voting."
"Casting a ballot and having your voice heard is an empowering experience that can really help people to recover from mental ill health.”