The EU Commission has told EU states to tighten checks on non-EU nationals who acquire citizenship - so-called "golden passports" - through investments.
The Commission plans closer monitoring of those schemes and of "golden visas" granting residence in exchange for big investments. It says they can be abused for tax evasion and money-laundering.
EU citizenship gives an individual free movement in most of the EU, easy access to the single market and other rights.
Twenty EU countries have such schemes.
Cyprus, Malta,and Bulgaria give passports to non-EU nationals who make sufficient investments in their countries. Rich foreigners can buy passports there for between €1m (£870,000; $1.1m) and €2m.
They and 17 other EU member states, including the UK, also grant residence rights to investors. That right puts an individual on the path to citizenship.