World boxing champion Vladimir Klitschko and his American actress girlfriend Hayden Panettiere have separated after 18 months together, they announced on Thursday. “We realise how difficult it is to maintain a relationship when living on two continents,” said the 35-year-old Klitschko, who divides his time between his homes in Hamburg and Florida. “I cherish Hayden as a person and a woman. I’m sure we will stay friends,”he said.