Npower was today fined £1.8 million for failing to take sufficient action to prevent misselling of energy contracts to customers.
The fine from the energy regulator Ofgem comes after heavy criticism of all energy firms over door stop selling of gas and electricity.
The regulator found Npower failed to take adequate steps after complaints from customers about visits by the companys doorstep salespeople.
Ofgem found while Npower had complaints procedures in place, they were not proactively followed and so cases of misselling went unchecked.
Sarah Harrison, Ofgem managing director of corporate affairs, said: This decision sends a clear message to energy suppliers that failing customers... will lead to Ofgem action, as well as associated reputational damage.
However, she did not go as far as to condemn doorstep selling of energy completely.
Misselling undermines consumer confidence, but getting it right on the doorstep can help customers make effective choices in the energy market, Ms Harrision said.
This is why Ofgems energy supply market probe committed to strengthening the doorstep selling rules.
The £1.8 million fine would have been higher but Npower promised to move quickly on its failings.