The group, one of the UK's "big six" energy suppliers, said its overall revenues were down 7% because of the decline in the UK.
E.On also said that the lower value of the pound since the Brexit referendum vote had caused "currency-translation effects".
E.On has about 4.6 million UK customers but did not say how many it had lost.
Adjusted profit for the first three months of the year - before interest and tax - was just over 1bn euros (£843m), down 34% on the same period a year earlier.
After interest and taxes, its adjusted net income was about 525m euros, a fall of 20%.
The fall in the value of sterling means that E.On's UK sales are worth less when converted into euros.
Chief financial officer Marc Spieker said the first-quarter performance was "in line with our expectations despite a difficult business environment".
In March, E.On reported a record €16bn (£13.9bn) full-year loss for 2016, mainly reflecting its move away from generating electricity from fossil fuels and a greater focus on renewable energy.
A fall in the number of UK customers has hit quarterly profits at German energy giant E.On.