Skype's internet telephony service is back up and running after a two-day blackout on Thursday and Friday and patchy recovery over the weekend.

The company has published a statement on its website detailing the causes of the disruption, denying that the problems were caused by a deliberate denial of service attack. The assertion was corroborated by independent security analysts.

Meanwhile, traditional telcos are gloating over the damage the outage has caused to Skype's brand and to similar ultra low-cost internet telephony services which threaten to undermine their revenues.

"I hate to say I told you so," a product manager at one UK-based telco told vnunet.com.

"Free VoIP services are fine for consumers but you should not rely on them for business use. They do not yet offer the reliability or quality of carrier-grade services."

Independent telecoms analysts agree with the telcos. "There is still a danger that services designed to be highly disruptive to traditional telecoms business models have been developed without sufficient regard for resilience," said Mark Main, senior analyst at Ovum.

"This is something we have been saying since consumer VoIP came to the fore during 2003.

"Telecoms engineering is no different to any other product development: there is always a commercial penalty to pay by compromising reliability or quality. You still broadly get what you pay for in telecoms."

Main pointed to anecdotal evidence suggesting that Skype's service had been degrading lately.

Skype's statement blamed the outage on a huge reboot of its servers following security patching and "a previously unseen software bug within the network resource allocation algorithm". The problem has been since been fixed.

Skype has been providing internet telephony services for nearly four years without serious disruption. The company claims to have up to 50 million regular users worldwide.