The translation of speeches into sign language at Nelson Mandela's memorial service was seen as a sign of inclusiveness, but it appears the sign language interpreter was a fake, according to several expert organisations.

Braam Jordaan, a deaf South African and board member of the World Deaf Federation, said the interpreter was making up signs as he went along. "The structure of his hand, facial expressions and the body movements did not follow what the speaker was saying," Jordaan said.

The man who signed for a section of the ceremony, including Barack Obama's speech, was simply making up his own signs, he said.

"I was really upset and humiliated," Jordaan told the SBS news website. "He made up his own signs. What happened at the memorial service is truly disgraceful thing to see – it should not happen at all. What happened today will be forever aligned with Nelson Mandela and the deaf community, thanks to this fake interpreter."

Members of South Africa's deaf community have previously raised concerns about the interpreter, who has been used at other African National Congress events. Despite this an ANC spokesman said on Wednesday: "I don't know this guy. He doesn't work for the ANC. It was a government event."

Collins Chabane, a government minister, told a news conference: "The government is looking in to the matter but has not been able to conclude this inquiry due to the demanding schedule of organising events."

A spokesman for the president, Jacob Zuma, said he was checking the reports, while the SABC state broadcaster, which covered the memorial, said it was not involved as it had its own on-screen signers.

During the interpreter's appearance on Tuesday, Wilma Newhoudt-Druchen, the first deaf woman to be elected to the South African parliament tweeted: "ANC-linked interpreter on the stage with dep president of ANC is signing rubbish. He cannot sign. Please get him off."

South African sign language interpreter Francois Deysel added on Twitter that the interpreter was "making a mockery of our profession".

Sheena Walters, of the World Association of Sign Language Interpreters, noted that the sign language the interpreter was using was unconventional at best and not recognisable as any accepted form of international or South African sign language. "It seems quite obvious that the interpreter isn't using South African sign language," she told SBS.

"Most sign languages across the world share a similar structure and pattern and this person seems to be making a lot of repetitive signs and isn't displaying the usual facial expression or structure of sign language that you would normally see."

Deaf news blog The Limping Chicken agreed and criticised the interpreter's methods of communication. He signed with a "strange repetitive rhythm to his movements" and "the structure of his hand and body movements didn't seem to change no matter what the speaker was saying", they wrote.