The European Commission is taking the action to prevent the "horrible shocks" experienced by many users of wireless dongles and phones who are unaware of data download limits and when they exceed them.
From today customers can oblige mobile phone operators to cut them off after a pre-set amount, and warn them when they have used 80 per cent of their entitlement.
If they do not set a limit, the new law requires the operator not to allow the bill to exceed €50 a month without warning.
The €50 limit runs until July, by which time the Commission expects users to establish their own spending ceilings.
The Commission said it will impose sanctions on operators if they flout the new regulations.