The care available for adults in Europe with congenital heart disease is inadequate, experts have concluded.
The researchers make recommendations to improve heart care
Better treatment for children with heart disease has meant increasing numbers are surviving.
But this research, published in the European Heart Journal, found there were too few specialist centres to support adults with the condition.
British heart experts welcomed the study, but said more work was needed to determine patients' needs.
The finding is the latest conclusion from the Euro Heart Survey on Adult Congenital Heart Disease, which has looked at how care is being organised in Europe.
Researchers analysed data from 71 centres in four countries which agreed to fill in questionnaires.
Three specialist centres and one non-specialist centre in the UK took part in the study.
The researchers say a range of eight measures should be in place to provide top quality care for adults with heart diseases.
These include having at least one heart specialist and one nurse trained to provide care to this group at each centre, and for services to be linked in with paediatric care.
In addition, surgeons should carry out a set number of operations per year on people with adult congenital heart disease.
The researchers found that less than a fifth of the 48 specialist centres fulfilled all eight recommendations.
And of the 23 non-specialist centres, only 14 formally collaborated with a specialist centre.
The two key areas that were most difficult for centres to comply with were performing the minimum number of congenital heart operations a year and involving nurse specialists in patient care.
However, because the researchers did not receive information for all centres in every country, they said they could not compare individual country's services.
But Dr Philip Moons, assistant professor at the Centre for Health Services and Nursing Research of the Catholic University of Leuven in Belgium, who led the research said: "We can definitely say that the provision of care overall is suboptimal and there is much room for improvement.
"As participation in the survey was voluntary, it's likely that only the most motivated and active centres completed our questionnaire, so our results may actually mask the real situation.
"Certainly, our findings suggest that the number of adequately equipped centres is too limited to support the more than 1.2m adults with congenital heart disease in Europe."
He said governments, health ministers and healthcare providers were under an obligation to provide adequate human and financial resources to meet the increasing needs of adults with congenital heart disease.
Dr Moons added: "Society has invested a lot towards increasing the life expectancy of these children, but seems less interested when they are grown up.
"If we are fully to realise the benefits of the cardiac surgery that can now be performed in infants and children, healthcare professionals must apply continuous effort to implement these recommendations."
Dr Mike Knapton, director of prevention and care at the British Heart Foundation said the study raised interesting issues.
"The survey finds that, whilst infants and children with congenital heart disease have excellent care in their early years, more financial support and provision of care is needed as they grow up to become adults.
"The report highlighted that more training and funding is needed for health professionals so they can continue to help and support adults with congenital heart disease.
"However, as the researchers themselves recognise, the survey was only carried out in four countries so further research would be needed to discover how services compare to other countries in Europe."