Ailing supermarket group Tesco came out at the wrong end of another set of sales data on the UK's food retailers on Friday. Tesco was the biggest sales faller of the 'big four' supermarkets in the 12 weeks to 8 November, dropping 4.2% versus the same period a year ago, according to figures from retail researchers Nielsen.Rival Morrisons was the next biggest victim with a sales fall of 3%, while Sainsbury's dropped 1.8% and Asda slipped 0.8%.The data follow figures from another research group, Kantar Worldpanel, earlier this week showing Tesco's sales fell 3.7% in the 12 weeks to 9 November, though the rate at which it was losing market share slowed.The established supermarkets have been struggling to keep up with discounters such as Aldi and Lidl, while chains such as Marks & Spencer and Waitrose have also been doing well at the higher end of the market.Aldi's sales leapt 21.4%, Lidl soared 23.3% and Waitrose rose 6.9% as they attracted new shoppers, while M&S was also up 1.8%.Overall, year-on-year grocery sales declined for the third month in a row in the four weeks to 8 November, falling 0.9% against the same month a year ago. Sales volumes dropped 1.2%.Nielsen predicted a drop in grocery sales for the final quarter of 2014 of between 0.5% to 1% against a year ago. For the first time in a decade, Nielsen projected a decline in overall industry sales across a calendar year - to £145bn from £146bn in 2013.Mike Watkins of Nielsen said: ""It's been a bad year for the big four supermarkets and they enter the key Christmas trading period with sales growth much weaker than 12 months ago."Much of this has been put down to the discounters, who are gaining share fast, but it's the needs and expectations of shoppers that have been changing - and some of the supermarkets simply haven't adapted quickly enough."