The National Planning Commission on Thursday rubbished the National Union of Metalworkers claim that DA economic policies formed the basis of the ANC's National Development Plan (NDP).
Earlier, Numsa, an affiliate of Cosatu, said its 311,000 members rejected the NDP as a "right wing" document.
"After a thorough analysis, the [central committee] came to the extremely disturbing conclusion that significant and strategic parts of the NDP were directly lifted from DA policy documents," said Numsa general secretary Irvin Jim.
He was briefing reporters in Johannesburg on decisions taken at the union's committee meeting.
The NPC responded on Thursday evening. "Mr Jim suffers from an infantile disorder that manifests as an acute aversion to anything rational," commission spokesman Dumisa Jele said in a statement.
"He arrogates to himself, as a custodian of ANC policies, power much greater than the 4500 delegates who gathered at the ANC 53rd National Conference in Mangaung," Jele said.
He asked: "What gives Mr Jim the right to suggest that the delegates were misled?"
The African National Congress declined to comment on the matter.
"We will take the matter with them offline as part of Cosatu, if there is a need or request from [the Congress of SA Trade Unions]," spokesman Keith Khoza said.
Democratic Alliance spokesman Mmusi Maimane said there was no plagiarism of DA policies "at all".
Because the NDP was the result of wide consultation "there are components which we completely agree with", he said.
"We certainly gave our inputs. We started implementing (aspects of the NDP) in the Western Cape.. [which] are proving fruit[ful].
"It affirms some of our policies and it is something we can do. For Numsa to take this position is unfortunate, because it creates policy uncertainty," said Maimane.
Jim said that Numsa also decided at the "painful" central committee meeting, that the leadership of the ANC, an alliance partner, as elected at Mangaung in December, was not "working-class oriented".
It had a businessman as a deputy president, said Jim, referring to Cyril Ramaphosa.
But, it would still encourage its members to support the ANC, and had no plans to form its own political party.