Hundreds of trading and investment firms from the City of London have set up shop in the EU to avoid disrupting business with the bloc by relocating staff and assets.
The costly investment was vindicated by an UK-EU trade deal that left UK financial services largely cut off from the continent after Britain left the EU.
The European Securities and Markets Authority (ESMA) had checked the licence applications from new hubs in case national regulators were offering sweeteners.
ESMA Chair Steven Maijoor said the watchdog has now begun reviewing how the licences are working on the ground in a process that will continue indefinitely to ensure sufficient activity and senior staff.
The close scrutiny will test post-Brexit relations between Britain and the bloc, already strained by clashes over Northern Ireland and COVID-19 vaccines.
Both sides aim to agree a cooperation pact in financial regulation by the end of March, a first step in restoring trust and potential UK market access further down the road.
A former regulator in the Netherlands, Maijoor is due to step down from ESMA after 10 years at the helm, having overseen a watchdog that has steadily increased its powers and reach.