The regulator said that funds being paid by existing and former domestic customers of the suppliers will be protected.
Customers of all three suppliers will be contacted in the coming days to be informed about the changes.
Neil Lawrence, Ofgem’s Director of Retail, said: “I am pleased to announce we have appointed Shell for the customers of Pure Planet, Daligas and Colorado Energy.
“We understand that this news may be unsettling for customers, however, they do not need to worry. Their energy supply will continue as normal and customer credit balances will be honoured.”