The financial watchdog, the FCA, fined Wonga £2.6m last summer after finding that 45,000 such letters had been sent out. It then referred the matter to the police.
Today the City of London Police issued a statement saying: "The central allegations were that Wonga had deceived its customers by sending letters falsely purporting to be from lawyers with the aim of recovering outstanding debts from customers.
"After a thorough review of all the material gathered the City of London Police has concluded there is not sufficient evidence to progress a criminal investigation. "