Shares in UnitedHealth Group, the leading US healthcare operator, fell sharply yesterday after Andrew Cuomo, the New York state attorney-general, said he would sue the company for allegedly defrauding consumers by manipulating reimbursement rates.

Following a six-month investigation, Mr Cuomo issued 16 subpoenas to the country's largest health insurance companies and said he planned to sue United-Health and a clutch of its subsidiaries centred on Ingenix, the US's largest provider of healthcare billing information.