Centrica, which owns the British Gas brand, has said it lost 382,000 energy accounts in the three months to May.
Customers continue to leave British Gas
Earlier this year, the firm had said it would focus on winning back customers it had lost in 2004.
However, on Monday the company said it "no longer expects to hold its energy product holdings flat in 2005".
Since 2003, the cost of gas for British Gas domestic customers has increased by 22%, while the price of electricity has risen by 18%.
Recently Centrica has been the subject of rumours surrounding a possible deal with Norwegian energy group Norsk Hydro.
Centrica warned that trading conditions could be more tricky through 2005 and 2006 if wholesale energy prices remain at current levels.
And, despite recent increases in prices for domestic customers, it flagged up further possible rises to protect its margins.
"Financially we have had an encouraging start to the year, but the winter period remains uncertain for all energy suppliers in the light of continued increases in forward wholesale commodity prices," said chief executive Roy Gardner in a statement.
"We will continue to take all the steps we can to mitigate the impact on our customers, but retail price rises across the whole of the industry cannot be ruled out," he added.
In February Centrica reported a 16% rise in operating profits to £1.2bn ($2.3bn).
Operating profits at the group's British Gas arm rose 64% to £337m - a figure that came in for criticism from consumer group Energywatch.
Sir Roy told shareholders that costs were being cut, and a new billing system is to be tested from the middle of the year.