London-listed supermarket stocks were lacking some festive cheer on Monday after discount grocer Lidl announced plans to more than double the number of stores it has in the UK.UK Managing Directr Ronny Gottschlich said that the UK grocery market is entering a "new era" as it aims to expand to 1,500 stores, up from around 600 stores currently.Rising competition from the so-called 'discounters' such as Aldi and Lidl has prompted concerns about market share at the 'Big Four' retail groups of Tesco, Sainsbury, Morrison and Asda over recent years, and this announcement will likely dampen sentiment further ahead of the key Christmas selling period.A number of analysts have said that there is now less and less differentiation between the Big Four's value ranges and the ranges of the discounters.Speaking to the Sunday Telegraph, Gottschlich said: "We really see ourselves these days more of a supermarket than the hard discounter of the past [...] Those times are over. We are now a supermarket. We offer the best quality for the least."In contrast with retail giant Tesco, which is expected to unveil a 1.5% fall in third-quarter like-for-like sales on Wednesday, Lidl's LFL sales are growing by 18% year-on-year, the paper said.Lidl, owned by German parent Schwarz Group, is said to be increasing its annual investment to £300m as it ramps up store openings from 15-20 to 30-40 per year.Tesco, which was hit by a downgrade from HSBC on Monday, was trading 1.94% lower by 15:00, while Morrison and Sainsbury were down 2.52% and 2.80%, respectively.BC