Scottish Power has lost 100,000 of its UK customers since the end of 2017, reducing the number of its consumers to five million.
The new data comes after the company introduced a hike in standard variable tariffs (SVTs), which is impacting the energy bills of 960,000 households and came into effect on the 1st June.
The company blamed government policy and an increase in wholesale gas prices for the rise in energy bills.
Scottish Power CEO, Keith Anderson, said: “The improvement in generation and supply follows a very difficult 2017, which delivered one of the weakest performances for the business in the last decade.
“The first three months of the year has seen the business recover to a level just below the first quarter in 2016.
“With the price cap pending this year, we still expect a challenging environment for the retail business in 2018.”