The service 11 88 88, operated by Irish firm Conduit under the name 118 Ltd, is currently running a TV campaign by Partners BDDH featuring three people singing 11 88 88 to the tune of "here we go, here we go, here we go".
There is no voiceover in the spot, but the endline runs 'Half the price of BT Directory Enquiries', a claim that has also appeared on poster ads.
BT contested the claim and Conduit withdrew the ads following the threat of legal action from its rival.
The news comes as another blow to Coinduit, which despite an ad budget of £40m, more than The Number's 118 118 and BT's 118 500, has won only 15% of the market replacing the 192 directory enquiry service.
The failure of the campaign to make a real market impact is thought to have contributed to chief executive Liam Young's resignation 10 days ago, and now the service faces running out of steam as reports at the weekend suggested that the Royal Bank of Scotland has refused to sanction a loan of €12.5m (£8.7m) to fund more ads.
Conduit's contentious campaign follows launch work featuring the numbers as cartoon superheroes, with a voiceover saying: "We'll save you" and "It's a job for 11 88 88".
The company has already been fined £15,000 by Independent Committee for the Supervision of Standards of Telephone Information Services (Icstis) over its pricing claims.
Paul Elliott, chief executive for BT Directories, said: "BT challenged Conduit's claim that 11 88 88 is half the price of BT Directory Enquiries because it is just not true. The only truth is that Conduit has been misleading customers."
He added: "When you take into account the full spectrum of calls to BT Directory Enquiries, as well as the range of our services for residential customers and businesses, Conduit has no grounds to claim its price is half ours. Therefore, the decision to withdraw the slogan from all its advertising is very welcome."
BT is also claiming that its service is more efficient than those of its rivals, following a poll from research firm ICM.
Elliot said that research BT had carried out into the length and quality of calls has shown that BT's 118 500 service beats 11 88 88 and 118 118.
"The findings showed that operators from both companies take far longer than BT's 118 500 service when answering like-for-like enquiries. People may have been enticed by Conduit's half-price claim but they were actually spending more than they thought because of the longer call-handling time," he said.
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