Related Stories
Tesco finalises China retail deal
Tesco to launch own-brand smartphone Listen
Tesco has reported a third consecutive quarter of falling sales as the UK's largest supermarket chain faces increasing competition from rivals.
For the three months to 24 May, the supermarket said like-for-like UK sales including VAT and excluding petrol fell by 3.7%.
Chief executive Philip Clarke said trading would remain challenging "throughout the coming quarters".
Tesco reported a 6% fall in annual profits in April.
The BBC's business editor Kamal Ahmed says that internally, Tesco has admitted the fall in sales is its worst performance for 20 years.
Tesco has faced intense competition from discount rivals including Aldi and Lidl as well as from more upmarket retailers such as Waitrose.
Industry figures continue to show the UK's largest supermarket suffering a decline in its share of the grocery market.
On Tuesday, data from grocery market analysts Kantar Worldpanel showed Tesco's market share declined to 29% in the three months to 25 May from 30.5% a year earlier, while sales fell 3.1%.
Tesco, along with Asda, Morrisons and Sainsbury's, has responded to the threat from discount grocers by lowering prices on essential items including bread, milk and eggs. Tesco said sales volumes of these essentials had risen 28% in the quarter.
It has also launched a programme of store revamps, which will see 650 stores across the UK upgraded in the next year.
Mr Clarke said: "Our accelerated plans are making a real difference for customers and we are more competitive than we have been for many years."