The US Chamber of Commerce, which numbers IBM Corp, Intel Corp and HewlettPackard Co among its members, warns that the European Union’s Working Hours Directive, which imposes a rigid 48-hour week poses the biggest threat to American investment in Ireland in 20 years, according to the Mail on Sunday. US firms employ 60,000 people in the Republic and many fear that inflexible work patterns may force them out. The warning comes in a Chamber of Commerce submission to the Dail, which is in process of passing the directive into Irish law.