The South African government has admitted that the sign language interpreter accused of gesticulating nonsense during the Nelson Mandela memorial service lacked qualifications and that the company who supplied him had a history of substandard services and in this instance, "cheating".

Hendrietta Bogopane-Zulu, the deputy minister for women, children and people with disabilities, conceded a mistake had been made, but denied that Thamsanqa Jantjie was a fraud and refused to rule out employing him again.

"It was bad. Was he a fake? No. Does he have the training? He has only the introduction to the training. That's like a lot of South Africans," Bogopane-Zulu said.

"It is the first time I've seen complaints come to my office from the deaf community about him. It was the first time yesterday that the deaf community had brought it to my attention."

Millions of TV viewers saw Jantjie, 34, interpreting speeches by Barack Obama and other global leaders at the FNB stadium in Johannesburg. His gestures baffled and angered deaf people around the world, with experts saying he did not know even basic signs such as "thank you" or "Mandela".

The African National Congress, having previously pleaded ignorance, said on Thursday that it had used Jantjie's services over the years, but was not involved in the organisation of the memorial service, and was therefore "not in a position to offer a view on how his services were secured by the government".

Jantjie claimed on Thursday that he is qualified but was hallucinating and hearing voices during the service, and that he is receiving treatment for schizophrenia.

"There was nothing I could do," Jantjie told South Africa's Star newspaper on Thursday. "I was alone in a very dangerous situation. I tried to control myself and not show the world what was going on. I am very sorry. It's the situation I found myself in."

Jantjie said his episode meant he was seeing things and hearing loud voices in his head, impairing his ability to hear and interpret the speeches. But he could not leave so he persevered. "Life is unfair. This illness is unfair. Anyone who doesn't understand this illness will think that I'm just making this up."

He did not know what triggered the attack, he added, saying he took medication for his schizophrenia.

Bogopane-Zulu argued that South African sign language had more than 100 dialects, making it impossible to be understood by everyone. "Unless there's something I'm missing, I don't think we as a country should say we're embarrassed. The issue of sign language has always been about where you live, what school you go to and what language you speak."

Pressed on whether South Africa should be embarrassed, she insisted: "I don't think it's the right choice of word. I don't think he was just picked up from the street. He went to a school for the deaf; I went to a school for the deaf."

Jantjie's first language is Xhosa, one of 11 official languages in South Africa, the minister continued. She said: "He was not able to translate from English to Xhosa to sign language. He started well and then in the middle he got tired and lost concentration. That did not mean he is a bad sign language interpreter."

Asked if anyone understood Jantjie's gestures at the memorial, she said only: "We will find someone who understands him, who requested his services, but we're not going to do it now."

Another interpreter at the event was similarly unable to participate when host Cyril Ramaphosa spoke in the Zulu language, Bogopane-Zulu said. She also said South Africa was well ahead of many other countries in providing sign language services for presidential speeches.

She admitted, however, that SA Interpreters, the company that provided Jantjie, had a poor record and had now "vanished into thin air. Over the years they have been getting away with this. They have been providing substandard services to clients. The company has been in existence for a while but it looks like they have been cheating."

Whereas the standard fee for an interpreter is 1,300 to 1,700 rand (£76-£100) a day, she noted, Jantjie was being paid just 800 rand a day.

She denied Jantjie had been a security risk and declined to comment on his state of mind. "I don't think it will get us anywhere to get into his health, his violence, his schizophrenia. I don't think other service providers or journalists there on the day had their health profiles discussed."

An investigation was ongoing, Bogopane-Zulu added. "Will we invite him to big national events in future? It's not for me to stand here and say yes or no."

One journalist asked if Jantjie would be "brought to justice". Bogopane-Zulu replied: "Why? What crime has he committed? Why should he be brought to justice? Yes, he did not sign as well as expected, but what crime has he committed?"

Bogopane-Zulu reiterated that sign language in South Africa lacked a universal standard and was the subject of disagreement among academics. "There is a battle between black and white sign language people. Urban and rural. Whose slang takes priority? What unit should be used to measure it?"

On Wednesday South Africa's leading deaf association denounced Jantjie as a fake, saying he was inventing signs, and described the episode as an insult to deaf people and Mandela himself.

Asked in a radio interview how he felt about being the centre of such scrutiny, Jantjie said: "It is very sad at this present moment because I believe that it was an issue that had to be dealt with earlier. If the Deaf Federation of South Africa have an issue with my interpreting it was supposed to be in clarity a long time ago, not during this crucial time for our country."

He added: "If I interpreted wrong, why is it an issue now? Why wasn't it an issue when I was doing interpretation at MaSisulu's funeral and many big events in South Africa?"

Jantjie claimed he worked for a company called "SA Interpreters" and when asked if he has a formal qualification, he replied: "Yes, absolutely."

The controversy has marred South Africa's 10-day farewell to Mandela, whose remains were lying in state for a second day on Thursday at the Union Buildings in Pretoria, where he was sworn in as the nation's first black president in 1994.