Claims must not appear again in current form, Advertising Standards Authority rules.
The UK Advertising Standards Authority (ASA) has rapped BT for misleading customers by promising the delivery of super-fast Infinity broadband in some areas and then failing to keep to its timetable.
The telecoms firm was slapped on the wrist following a number of complaints to the advertising watchdog.
The ASA said in a statement that the appearance of a date or provisional date in the availability checker suggested that active plans were in place to roll out BT Infinity in that area, and that the service would therefore be available on or around that time. However it found that in some cases, promised dates were not met.
"We noted we had not seen supporting documentation showing that, in each of the areas where the complainants were based, the listed date in the availability checker was based on scheduled plans for Openreach to have BT Infinity available in those areas on or around the listed dates," ASA said.
BT said in a statement that the ruling was disappointing. "People clearly want to know when fibre might become available in their area and so BT has been publishing its best estimates on a regular basis. Those plans sometimes have to change however if local planning permission isn't granted or we find that a third party can't meet our deadlines," BT said.
"15 complaints is a tiny number when you consider that BT is enabling tens of thousands of cabinets but we will work with the ASA to make changes where we can as it is clear that people remain keen to know when fibre might reach them."