The London-based Chartered Institute for Securities and Investment (CISI) has experienced a security breach that appears to have compromised some of its members’ bank account and credit card details.

“Earlier this week we were notified by some of our customers that some fraudulent activity appeared on their statements after having transacted with us through a website,” said an email sent to CISI members signed by John Preston, CISI’s chief operating officer (COO).

“Whilst at this stage, we cannot be certain that these suspicious transactions took place we are writing to everyone that has made a purchase or pay...