input: Article: 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.

Summarize.
output: A fall in the number of UK customers has hit quarterly profits at German energy giant E.On.
Article: Mark Garnier said he would welcome a costed business proposal but it was "very unlikely" taxpayers' money would be spent investigating the scheme.
Several Tory MPs have called for a new, publicly-owned yacht or for the last one, Britannia, to be brought back.
Mr Garnier ruled out recommissioning Britannia, which was taken out of service in 1997.
"Clearly it's well past its active life," he said.
The old yacht, used to transport the royal family and to promote UK trade and industry, was turned into a floating tourist attraction in Leith, Edinburgh.
Conservative MP Jake Berry, who has led the royal yacht campaign in Parliament, secured a debate on his plan in Westminster Hall.
"I believe if Brexit is going to mean successful Brexit, it should also mean the return of our royal yacht," he said.
Royal Yacht Britannia
Could Brexit bring back the Royal Yacht?
Mr Berry said no public funds should be committed to the project, saying private financing should be sought.
He said his preferred option would be a new yacht, rather than bringing back the old one, saying it would be built in Britain and would be "a thoroughly modern ship, reflecting a modern nation and in fact a modern monarchy which is willing and able to serve Britain across the globe".
Former defence minister Sir Gerald Howarth said seeing the Queen's sadness at the decommissioning of Britannia in 1997 was "one of the darker moments of my political life".
He added: "Brexit makes the building of a new royal yacht not a luxury but a must-have."
Sir Gerald said UK trade negotiators would be taken more seriously by their foreign counterparts if they arrived in a "brand new symbol of our newly reasserted national sovereignty", which he said would be "a statement of our national intent".
He said the yacht should be operated by Royal Navy personnel, with the costs this created split between the defence, business, foreign and international development departments.
But the SNP's Deirdre Brock dismissed the plan as a "wistful throwback", saying disadvantaged people were being "left to go cold and hungry", but "we will all be paying for what must seem to them like a new pleasure cruiser for the royal family".
Labour's shadow international trade secretary Barry Gardiner said the government should be focusing on the economic impact of Brexit, not "playing with toy boats as virility symbols".
Responding for the government, Mr Garnier, the international trade minister, said Britannia "was and is an iconic symbol".
No business case has been put forward by supporters of the plan, he said, saying the government would welcome one.
But he cast doubt on the trade benefits of the scheme, saying there was no conclusive evidence trade deals that were signed on board Britannia would not have been signed elsewhere.
Mr Garnier said there were "hard facts which stand in the way of a new yacht" - including the likely Â£120m cost and the need to fund its operation and maintenance.
"We have to be clear the government has no plans and has had no plans to commission a new royal yacht, and as such it is very, very unlikely indeed that the government will use taxpayers' money to fund a royal commission or an investigation into whether we could commission a new  royal yacht," he added.
++++++++++
Summarize: There are "no plans" to commission a royal yacht to spearhead the UK's post-Brexit trade talks, a minister says.
Problem: The industry is being targeted in a UK-wide campaign by HM Revenue and Customs (HMRC) aimed at recouping unpaid tax.
Officials say a big increase in online escort agencies has helped create an industry worth £5bn.
The UK tax authority said many of these businesses were paying the tax they owed but others hid from payments.
The HMRC taskforce will investigate both traders and entertainers, some earning thousands of pounds a day, who do not register for VAT, income tax and PAYE.
"The people being targeted by this taskforce have no intention of playing by the rules, and we won't tolerate this," said Jim Stevenson, head of HMRC taskforces.
"No industry is safe where tax evasion is concerned - we won't stop tracking people down and taking back what they owe."
The campaign follows a case in November in which a 43-year-old woman, who ran an escort agency, was jailed for three years after fraudulently claiming £118,000 in benefits and avoiding paying £230,772 in income tax and national insurance.
Janine Adeleke ran the business from her home in Bexhill-On-Sea, in East Sussex. During her trial at Canterbury Crown Court, the jury heard Adeleke lived in a house with eight bedrooms and sent her five children to private school.
At the trial, HMRC described her lifestyle as "lavish".
HMRC said that more than 140 taskforces have been launched since 2011 - targeting industries ranging from dentists to lawyers - which have collected more than £404m in unpaid tax.
What was that article about?

A: Strippers, online escort agencies and adult club owners have been told to reveal all when it comes to their tax affairs.
Question:
Article:Vasil Stoyanov, 30, was caught by Border Force officers with a lorry-load of the class A drug at Dover Eastern Docks in June 2015.
The National Crime Agency (NCA) said the haul was estimated to be worth £3m wholesale and £11m on the street.
Stoyanov, who had pleaded not guilty, was sentenced to 14 years in prison at Maidstone Crown Court on Friday.
Following an NCA investigation, a border force officer found 130g of the drug powdered and sealed in a plastic bag stuck to the inside wall of one of the shipment's 2,000 cardboard boxes.
A search of the lorry's load found 228 boxes had heroin inside concealed in the same way.
The shipment was addressed to Lizzy86 Ltd, Stoyanov's company which was exposed as a front.
Cambridgeshire police officers searched his home in Swinburne Close, Kettering, and a nearby lock-up.
They found cut open charcoal boxes and documents showing he had spent £66,000 on three large orders of charcoal for a return of £67.
His eBay account listed just two lots of charcoal for sale at £3.50 each.
Mark Harding, senior investigating officer at the NCA, said: "The seizure means serious organised criminals have lost out on a huge revenue stream and street-level dealers won't be able to endanger heroin users and the communities in which they live.
"It's also shut down a smuggling route that could have been used to bring any kind of dangerous commodity into the country."

Summarize the main points of that article.
Answer:

A man who tried to smuggle £11m worth of heroin into the UK by hiding it in boxes of charcoal has been jailed.