The Advertising Standards Authority (ASA) received 15 complaints from consumers who had used BT's online "availability checker" to see when they could get Infinity broadband based on their landline, phone number and address.
One complainant said that they had searched on September 20, 2012, and been presented with a table saying they could get BT's Total Broadband package now, and Infinity would become available on September 30.
But when they came back to the website and checked again, it said that Infinity broadband would not become available in their area until December 31, 2012.
There were 14 other complaints of a similar nature, accusing BT of being misleading in the way it presented the dates, as often the true availability had been pushed back "repeatedly" and in some cases there were no plans at all to bring fibre to that particular area or post code.
BT said that it had made changes on October 16 last year to its Infinity checker to reflect that the dates shown "were provisional". The firm said it therefore feels customers now get "sufficient information to prevent them being misled".
However, the ASA said that one complainant had been informed by BT on October 17 that plans for the BT Infinity roll-out in his area had been put on hold, but when he resubmitted his details his area was marked as December 31, 2012.
The ASA said that it had seen documentation that indicated the dates supplied to consumers via the availability checker were based on provisional plans by wholesale arm Openreach to expand BT Infinity into those areas.
"We considered that the date BT Infinity was expected to be available in their area would be a material consideration for consumers when deciding whether to register an interest in the service," the ASA said.
"Because we had not seen documentation that showed there were scheduled plans to roll out BT Infinity in the complainants' areas and make BT Infinity available in those areas on or around the listed dates, and because we understood that, in at least one case, there were currently no plans for BT Infinity to be rolled out in an area while the availability checker listed a provisional date that was less than three months away, we considered that the inclusion of provisional dates was likely to mislead."
The ASA recorded three breaches of its rules, and warned BT that the checker must not appear again in its current form.
The regulator also warned BT that it must ensure that any dates shown on future checkers only correspond to dates when Openreach are firmly scheduled to install BT Infinity in that particular area.
In its response to the ASA, the telecoms giant said that it is currently developing a new availability checker for delivery in early 2013, and would "bear in mind any issues raised about properly communicating availability dates".