LONDON, July 22 (Reuters) - Daily contact testing will be rolled out to workplaces in Britain"s food sector so staff who have been "pinged" by the COVID-19 app can keep working if they test negative rather than isolating, the government said on Thursday. Some supermarkets are facing shortages of specific products - mainly those in demand in hot weather - and some petrol stations have had to close after the health app told workers to isolate following contact with someone with the virus. British newspapers carried front-page pictures of empty shelves in supermarkets, declaring a "pingdemic". With cases rising to nearly 50,000 a day in the United Kingdom, hundreds of thousands of people have been advised - or "pinged" - by the National Health Service"s contact-tracing app to isolate for 10 days. The government said priority testing sites would be set up at the largest supermarket distribution centres this week, and up to 500 sites would start next week. "As we manage this virus and do everything we can to break chains of transmission, daily contact testing of workers in this vital sector will help to minimise the disruption caused by rising cases in the coming weeks, while ensuring workers are not put at risk," Health Secretary Sajid Javid said. Contact testing is designed to limit the spread of COVID-19 by identifying people infected by being in proximity to someone with the virus, either via the app or human contact tracers, but who have no symptoms themselves. WARM WEATHER LINES The government said supermarket bosses had downplayed fears of shortages, saying problems were not widespread. Sainsbury"s (SBRY.L), Britain"s second largest grocer, said customers may not be able to find the exact product they want. "Large quantities of products are being delivered to stores daily and our colleagues are focused on getting them onto the shelves as quickly as they can," a spokesperson said. Asda, the industry"s No. 3 player, said although more staff were being forced to self-isolate the level of absences remained well below what it saw at the height of the pandemic last year. "We’re also not in a position where we would have to close any stores," said a spokesperson. Product availability was "largely OK across the board" though some stores were short of warm weather lines: beers and soft drinks, bottled water, ice cream and salad leaves. But smaller rival Iceland said it had closed a number of stores due to staff shortages. BP (BP.L) said it had to temporarily close a handful of petrol stations due to a lack of fuel, with a shortage of HGV drivers exacerbated by COVID-19 isolations. Official data showed the app had told nearly 620,000 people to isolate in England and Wales in the week up to July 14. Britain has allowed some workers in critical roles to carry on working, even when "pinged". Companies must apply for authorisation for their staff, and it will only be granted to designated workers in approved workplaces who are fully vaccinated, the government said on Thursday. Infections had been rising in Britain for several weeks, but a vaccination programme appears to have weakened the link between infections and deaths, with daily fatalities remaining relatively low.