Broadband 'blackspots' must be revealed to street level to end the digital divide, a committee of MPs demands today.
BT and the government are sharply criticised for the lack of information about rural areas that will miss out on speeds of 24mbps - making it impossible to "plug the gaps".
Vague maps have prevented rival broadband firms bidding for cash to supply the hardest-to-reach premises, the Commons public accounts committee (PAC) says.
Its report blames confusion about the data local councils are allowed to release from BT, the monopoly supplier of the main broadband contracts. It urges the department for culture, media and sport (DCMS) to end that uncertainty - by requiring openness "down to full (seven-digit) postcode level".
Margaret Hodge, the PAC's Labour chairwoman, said: "Communities can still not access the detailed data they need to understand whether they will be covered by BT's scheme in their area. Other broadband providers might be squeezed out of the rural market by BT's actions."
Last year, the government ripped up a deadline of May 2015, the date of the next general election, to deliver 24Mbps speeds to 90% of homes and offices in each area.
With just nine of 44 sub-regional projects on track, ministers were forced to replace the target with a pledge to extend superfast speeds to 95% of homes, by the end of 2017.
They then announced a £10m pot that will exploit new technologies to deliver superfast speeds to "absolutely everybody", but no date has been set.
Local authorities have protested that they are unable to pursue plugging the gaps because BT has refused to allow them to release details of blackspots to potential rivals.
Today's report quotes the example of Cumbria, but says the DCMS had insisted the county council "was able to publish the data it wanted to".
The PAC also repeated criticism of a lack of "meaningful competition", with BT snapping up all 44 of the sub-regional contracts for the £530m scheme.
That had led to BT "exploiting its monopoly position to the detriment of the taxpayer, local authorities and those seeking to access high speed broadband in rural areas".
Hodge said ministers and officials would be hauled back before her committee if they failed to ensure publication of broadband data.
But communications minister Ed Vaizey said: "Britain has the best superfast broadband of all five leading European economies. The rollout is ahead of schedule. We continue to promote competition and have insisted on enhanced transparency from both local authorities and suppliers."