Novartis shares have fallen after US regulators said a unit that developed a costly breakthrough gene therapy knew about potential manipulation of animal testing data but failed to disclose it before the treatment was approved.

Novartis, the Swiss drugmaker which employs 1,600 people in Ireland, has agreed to acquire AveXis for $8.7bn (€7.1bn) to gain a promising drug to treat a rare disease that afflicts infants, hastening a shift towards gene therapy and precision medicines.