Visa has warned large merchants that they will face fines and higher credit card transaction fees unless they comply with the Payment Card Industry (PCI) data security standard by 1 October.

Visa sent that message to banks that process card transactions for merchants. These "acquiring banks," which are charged with enforcing the PCI requirements, passed the warning on to their clients.

The PCI standard was developed by several credit card firms, which began pushing merchants two years to begin work on meeting its 12 requirements.

Visa warned last December, and then reiterated this month, that merchants failing to comply by the October deadline will face higher commission fees and monthly fines of at least $25,000.