The commission was asked by the Office of Fair Trading last April to investigate the market for the third time since 1995. It has previously imposed below inflation price increases on Yellow Pages, which was then owned by BT and now by Yell Group.
This week, it released its latest thinking and Diana Guy, the inquiry group chairwoman, said: "From the evidence so far presented it appears that, despite developments in this market, Yell still holds a strong position."
Yell is estimated to have a market share of around 84%, ahead of Thomson Directories, BT, which re-entered the market in 2003, and Trinity Mirror.
The commission's statement was criticised by Yell, particularly for its conclusion that it will continue to regard internet classified advertising and printed directory classified advertising as separate markets.
The commission is required to publish its final report by April 4 2007 but expects to report well before that date.
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