25,000 TalkTalk customers switched to a different provider in the final part of 2010.
TalkTalk broadband shed 25,000 customers over the last quarter of 2010, though it still has more than 4.224 million customers, its latest results have revealed.
TalkTalk put the drop in numbers down to some former Tiscali customers switching broadband provider following their migration to its own network.
The fall in customer numbers means that it could have slipped into third place behind BT broadband and Virgin Media.
Last week BT latest figures showed it had 5.5 million broadband customers, and although Virgin Media won't be releasing its figures until 17 February, its last results showed that it had 4.24 million customers.
TalkTalk said that it was still "on track" though, having unbundled more than 85% of its broadband network.
"We are now making good progress towards our goal of a further 700 unbundled exchanges covering 93% of the population," added TalkTalk.
The provider, which offers some of the cheapest broadband and home phone deals on the market, also revealed that it is set to trial fibre-to-the-home (FTTH) broadband through the BT Infinity network.
By running fibre direct to homes in north-west London, the FTTH trial will offer speeds of up to 100Mb broadband.
This could eventually complement the new Fibre Optic Boost, which is due for a full launch over coming months.
TalkTalk said: "The Boost will deliver download speeds of 15-40Mb for customers seeking faster broadband access, and will make a real value-for-money, high-speed broadband service available to our customers."