LONDON, May 20 (Reuters) - British manufacturers reported the fastest growth in orders since December 2017 in May but feared that supply chain bottlenecks would cause their costs to rise by the most in over three years. The Confederation of British Industry said its monthly industrial orders balance - which measures the difference between the proportion of employers who say order levels are above or below normal - rose to +17 in May from -8 in April. This is the highest since December 2017 and well above forecasts of an improvement to zero in a Reuters poll of economists. The index for output growth over the past three months was the highest since December 2018 and the CBI said it represented the first material growth in almost two years. CBI economist Anna Leach said businesses were optimistic but had concerns about supply bottlenecks, in part caused by continued requirements for social distancing at work as well as broader disruption to global trade from the COVID pandemic. “Firms are still feeling the chill as supply shortages fuel cost pressures, reflected in expectations for strong output price inflation in the coming quarter,” she said. The survey’s price balance rose to +38 from +27, its highest since January 2018 and well above its average of +3. (Reporting by David Milliken Editing by William Schomberg) Our Standards: The Thomson Reuters Trust Principles.