Mobile operator and ISP O2 UK ( Be Broadband ) has released its latest quarterly results to the end of September 2011 (Q3-2011), which revealed that the providers rate of broadband subscriber decline has accelerated with the loss of a further -27,600 customers to total 625,300 (-6.0% year-on-year).

The worrying figures suggest that O2 has been unable to turn its misfortunes around and were sharply down for a third consecutive quarter, having previously lost -16,300 customers in Q2 and -2,400 in Q1.

The figures do not take into account any adjustment caused by O2's recent return to offering "truly unlimited downloads" (here) because that occurred in September 2011 (though they still threatened to disconnect excessive users). Sadly O2's parent, Telefonica, does not offer a breakdown of O2 and BE subscribers so we cannot see how the two sides of their ISP business compare.

O2 continues to be one of the highest rated providers amongst the big boys and the level of service complaints do not appear to have increased. It could be that earlier price hikes and the ISPs lack of a competing superfast broadband product, which several rivals now have, might be eating into their market share but it's hard to tell.

Aggressive pricing competition from Sky Broadband and TalkTalk can't be helping, although TalkTalk has had its own problems with subscriber declines due to poor customer service, billing errors and various other things that Ofcom has held them up for during 2011. By contrast Virgin Media , BT and Sky have seen most of the quarter's growth.