LONDON (Reuters) - Prime Minister Boris Johnson urged Britons to be cautious as a stay-at-home order and some other lockdown measures are lifted in England, citing rising cases in other parts of Europe and the threat posed by new variants of the virus. From Monday, up to six people, or two households, in England can meet outside while outdoor sporting facilities such as tennis and basketball courts can be used with social contact limits in place. Britain’s vaccination programme has been one of the most successful in the world but there are concerns over supplies, high infection rates in parts of Europe and the ongoing emergence of new coronavirus variants. “We must remain cautious, with cases rising across Europe and new variants threatening our vaccine rollout,” Johnson said. “Despite today’s easements, everyone must continue to stick to the rules, remember hands, face, space, and come forward for a vaccine when called.”