Diageo , the UK food and drinks group, said on Thursday that trading at its Burger King fast food chain remains weak. The group warned that full-year operating profit for the hamburger chain was likely to be down year-on-year. "Trading at Burger King remains challenging and in January comparable restaurant sales were down approximately three percent, however February is slightly better...the full year reported operating profit is likely to be down year on year," Diageo finance director Nick Rose told a conference call to analysts.