Hawksmoor Investment Managemet revealed it had been the victim of a ‘phishing’ email scam on Tuesday.
The firm, which has offices in London, Exeter, and Taunton, reassured customers that their private information was not compromised.
And bosses vowed to stop it reoccurring in future.
Hawksmoor chief executive John Crowley wrote to clients to inform them of the problem.
Mr Crowley said: “Hawksmoor was the target of a ‘phishing’ email attack on its IT systems on Tuesday, which our IT team rapidly identified and resolved, taking immediate steps to prevent this kind of incident from happening again.
“I'm writing to offer my fulsome apologies for any inconvenience caused to you by emails you may have received earlier this week purporting to be from Jill Gill, our business development manager.”
An email claiming to be from Ms Gill contained a Drop Box link for recipients to click through to – but it was bogus.
Mr Crowley said: “Hawksmoor treats confidentiality and data protection extremely seriously, and I can assure you that no client data was compromised at any stage.
“So, if you received any emails from Jill Gill between about 3pm and 4pm on Tuesday 17th January 2017, please make sure you delete them.
“If you opened any link on those emails and were asked for any information, such as usernames or passwords, it is the usual practice to take the precaution of changing these credentials immediately. I would recommend this.”
He added: “Once again, my sincere apologies and if you would like to discuss this incident further, please don't hesitate to get in touch with me.”