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Spark has set up a hotline following reports of customers receiving "aggressive" phone calls from scammers.
The callers claimed to be from Spark and told customers their services would be cut off due to security issues unless they changed their settings or cleaned up their computers, Spark head of corporate communications Richard Llewellyn said.
"The customers are then directed to a website where they are asked to download some software, which the fraudsters have branded as Spark."
Customers were also being asked to give remote access to their computer, provide personal banking information and leave their phone off the hook for three days following a "setting change", Mr Llewellyn said.
"Spark understands that in some instances the fraudsters are giving customers a made-up "employee number" to verify their identity."
Spark manager of channel operations Richard Harrison said it was important customers did not visit the fraudulent webpage, or provide any personal banking information.
"If they have any doubts about a phone call's legitimacy, they should end the call, and ring Spark's new scam helpline, on 123, option 9, to check.
"These fraudsters appear to be actively monitoring our activity and copying our methods of communication, so we know it can be difficult for customers to tell the difference between genuine calls and fake ones."
Spark would never ring customers out of the blue and ask for personal information, particularly bank details, Mr Harrison said.
"We will only ask you for personal information if you call us."
Anyone who suspected they had been the victim of a scam and may have passed on banking information or logged on to online banking during one of those calls should contact their bank immediately.
By Holly Bagge