Energy firm SSE expects a decline in profits from its energy-supply division after it announced it had lost 90,000 customers in the quarter to 30 June 2015.
In the three-month period, the company's customer base had shrunk from 8.58 million to 8.49 million households as market competition increases.
Alistair Phillips-Davies, SSE chief executive, said: "While there have been significant developments affecting all parts of the business, there has also been important progress in operational performance, which is essential for meeting the expectations of customers now and in the future."
The company said that it still expects a profit rise in its energy-portfolio management and electricity-generation division in its 2015-16 financial year, but it adjusted its forecast to a decline in energy supply profit.
Customers are increasingly switching to providers other than the Big Six, which contradicts a report by the Conduct and Markets Authority claiming that customers are overcharged millions but are not informed about the possibilities of switching providers.
Apart from that, SSE is also struggling to compete with market-leader British Gas, which announced its second 5% gas-price cut in six months on 15 July.
Higher levels of scuritiny from regulators Ofgem and the CMA, as well as increasing pressure from the already highly competitive market, are proving to be challenges for the energy providers.