The Ofgem investigation will take months, not weeks
Npower has dismissed six members of its sales team after claims of mis-selling energy contracts.
The firm had suspended 17 members of staff after an undercover reporter said some had misled potential customers.
The company added that the dismissals were "subject to due process and the rights of individuals to appeal".
Regulator Ofgem has also started its own probe into the claims. Rules state that customers must be aware they are entering into a contract.
Ofgem's probe is the first for alleged mis-selling since London Electricity, now part of the EDF group, was fined £2m by the regulator in 2002 after being found guilty of breaching the rules.
Since then, the watchdog - which has powers to impose fines of up to 10% of global turnover - says that complaints about mis-selling have dropped significantly.
Npower said it acted promptly after the claims. All its sales staff have been reminded about the rules, and mystery shopper schemes have been set up.
A spokesperson for Npower said it would be continuing an internal audit of sales practices.