UK supermarkets Tesco, Morrisons, Asda and Sainsbury could be forced to close some of their stores, Waitrose chief Mark Price has warned.Price, who has been notably vocal about the current picture on the grocery market in recent times, said it would be "incredibly hard to call" whether the 'big four' supermarkets would survive the difficulties facing the sector.Traditional retailers have struggled to keep up with the growing popularity of discount supermarkets such as Aldi and Lidl, which have both dramatically increased their market share as that of the big four has dwindled.Speaking in an interview with the Telegraph at a Waitrose store in Salisbury, Price said: "This is as fundamental as supermarkets coming into the UK in the 1950s and reinventing what food shopping was all about."I think we are at one of those inflection points where customers are acting differently and retailers are going to have to respond to it."Look at B&Q. Look at Homebase. I think that food is probably four or five years behind non-food. What you have seen over the last five years is 12% of non-food space taken out of the market. "You have had no food space retired over that period."In fact what you have been seeing is food space growing by 3-5%. So, more and more space has been added at a time before you get the impact of internet, convenience shopping and all the other shifts that we talked about."