TalkTalk has issued an apology for the troubled transition of Tiscali customers onto its own systems following its acquisition of the company in 2009.

Major problems had emerged during the switchover, and thousands of customers complained they were being billed for services they had not used. TalkTalk in March repaid nearly £2.5 million to customers - an average of around £40 each.

Charles Dunstone, TalkTalk chairman, told investors in the company's newly published annual report this week that migrating customers onto its own network and billing system was "an enormous and complex operation".

"The process of absorbing Tiscali's customers into TalkTalk was not always painless, and some suffered disruption to their service," he said. "We sometimes did not deal with these problems as well as we should have done, and this resulted in some customers getting frustrated with us and leaving TalkTalk."

Dunstone insisted, however, that TalkTalk now had a "stronger, integrated business". He added: "I am certain that for those customers who stayed with TalkTalk, and they are the vast majority, the journey was worthwhile and they will now enjoy a consistently better customer experience."

TalkTalk at one point was running nine billing systems, following the acquisition of Tiscali and OneTel. The company's own 'Trio' (The Right Information Online) platform has now replaced all billing systems in the group. Trio is understood to be based on technology from suppliers Chordiant and Tibco.

In May, TalkTalk said the Tiscali integration had achieved its target of £55 million cost savings.