Internal disciplinary hearings by Npower will end on Friday following allegations of mis-selling by some of its staff.
Four people from the company's sales team have already been dismissed during an internal investigation into claims that energy contracts were mis-sold.
Npower suspended 17 members of staff after an undercover reporter said some staff misled potential customers.
Regulator Ofgem has also started its own investigation 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, Ofgem - 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.
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