The new legislation by ESMA - MiFID II came into effect on the 3rd of January 2018. As a result, regulated and reliable brokers such as Trading 212, will need to confirm your nationality if you want to trade shares or other stock market listed instruments.
For example, solely UK nationals need to confirm their nationality by providing their National Insurance Number (NiN). If you have both British and Dutch nationality, we require both your UK National Insurance number and your Netherlands National Passport Number or a National Identity Card number (if you don't have a National Passport)