Tiscali UK business customers were left stranded on Friday when their leased lines were cut off without any notice - and all because the ISP had forgotten to pay a bill.
As many as a dozen companies were hit by the outage after BT cut off the leased lines for non-payment.
Tiscali UK resells BT leased lines.
Those hit were former Nacamar customers - an IP company Tiscali acquired as part of its purchase of World Online in December 2000.
One insider said the lines were down for at least six hours and that Tiscali was unaware of the problem until angry customers began phoning up to complain.
In a grovelling statement the ISP said: "Tiscali UK would like to sincerely apologise to the leased line customers for any disruption they may have experienced recently to their services.
"The integration of World Online, Liberty Surf and LineOne inevitably necessitates a transitional period for Tiscali UK. Whilst the company goes through intensive change, systems, processes and procedures have to be identified and rationalised across all acquisitions.
"Despite our best attempts to make this transitional period as seamless for our customers as possible, some of our leased line customers experienced a disruption to their services.
"During the integration of the companies, movement of staff and restructuring, a small number of leased lines from one of our suppliers was inadvertently terminated, which resulted in some of our customers losing service for a short period of time," it said.