DOVER, England—More countries closed their borders to Britain on Monday over fears of a highly infectious new coronavirus strain, heightening global panic, causing travel chaos and raising the prospect of UK food shortages just days before the Brexit cliff edge. India, Poland, Switzerland, Russia and Hong Kong suspended travel for Britons after Prime Minister Boris Johnson warned that a mutated variant of the virus up to 70% more infectious had been identified in the country, while Japan and South Korea said they were monitoring the situation. A slew of countries have already suspended travel, including France, Germany, Italy, the Netherlands, Austria, Ireland, Belgium, Israel and Canada. The discovery of the new strain, just months before vaccines are expected to be widely available, sowed fresh panic in a pandemic that has killed about 1.7 million people worldwide and more than 67,000 in Britain. Australia said two people who travelled from the …