Prospect national secretary Ben Marshall, representing more than 20,000 managers and professionals in BT, said that the company does not have a monopoly on broadband provision and faces stiff competition from numerous providers.
"Competitors like Virgin Media and Sky purportedly offer broadband throughout the UK, but simply choose not to get involved in providing it in unprofitable areas, and there is always the opportunity for others, eg Fujitsu, to get involved," he said.
"But these firms' business model – which largely relies on making a relatively rapid return – means that they feel they have no incentive."
On the other hand, he said, BT was willing to invest in the long haul. "Broadband overall will only make meaningful returns for BT over 11 years on average, and for most of UK plc that's simply too long.
"BT is being criticised effectively for being the only private company willing to provide broadband in places where by any normal measure doing so makes no commercial sense. Nor is BT a poor corporate citizen.
"It pays its taxes, pays the London and national living wage (or often significantly more), recognises trade unions and enacts fair procurement policies."
Marshall stressed: "As a union, we certainly don't think the company is perfect. But its record is as good as many and better than most. For a Labour politician to criticise BT remorselessly is potentially self-defeating as well as unfair and demoralising for BT's workforce."