The pledge was made by former prime minister David Cameron on a visit to Aberdeen in January 2016 as the industry struggled amid the oil price crash.
At the time the UK government said the post would “help ensure the best possible access for UK companies to markets overseas, promote the North Sea around the world and boost inward investment”.
However it now appears that no such appointment has been made, with government sources describing it as an initiative from Mr Cameron and the previous government.
On a visit to Aberdeen, UK energy minister Richard Harrington told the Press and Journal newspaper the ambassador role sounded like a “good idea” but he was “not aware” of the promise, adding: “It’s not crossed my desk”.
Kirsty Blackman, SNP MP for Aberdeen North, said the revelation was “deeply, deeply embarrassing” for the Tory party under Theresa May.
“They’ve been caught red-handed,” she said.
“Not only have they broken promises made to the oil and gas industry in its hour of need, they’ve conspired to brush their inaction under the carpet.
“It’s a display of shameful contempt towards the sector and to workers who have lost their jobs.
“When the prime minister sallied forth on a day trip to Aberdeen last January he was simply after a quick headline, not serious about measures to revive one of our key industries and the economy of the north east.”
The UK government said four business ambassadors have a focus on the energy sector as part of the Department for International Trade business ambassador network.
Mr Harrington said: “These are challenging but exciting times with new opportunities in North Sea oil and gas.
“We are working with the sector to build on the £2.3 billion worth of UK Government support through our modern Industrial Strategy.
“I want to make it clear that the industry has the full support of the UK Government, and that we are continuing to create the right environment though a stable and supportive package to allow business, enterprise and jobs to flourish. “