The independent company said it now supplies more than 550,000 customers, most with both gas and electricity, equalling more than 1 million accounts.
First Utility says it now holds a 2 per cent share of the retail market, compared to 92 per cent held by the big six.
However, the overwhelming majority stake held by the incumbent providers is down from 96 per cent from 2011, which First Utility says is down to the smaller providers’ ability to offer “lower energy prices and better value for the UK consumer”.
Trade association Energy UK said the milestone proves how increasingly competitive the retail energy market has become.
“More than 225,000 households are switching suppliers each month, and are shopping around to find the best supplier to suit them,” a statement from the group said, adding that consumers now have a choice of over 20 energy suppliers.
To mark its customer milestone First Utility said it will donate 1 per cent of its profits annually to a new foundation aimed at supporting families and individuals who are vulnerable to fuel poverty.
“The First Utility Foundation builds on First Utility’s existing community engagement programmes, but will act on an independent basis,” the company added.