JOHANNESBURG (AFP) - Two directors of the company that employed a bogus sign language interpreter for Nelson Mandela's memorial service have quit their jobs in South Africa's ruling African National Congress (ANC), a party spokesman said on Tuesday.

Reverend Bantubahle Xozwa, who was head of the party's religious affairs department, and his wife Cikizwa, a personal assistant to the ANC's chief spokesman, resigned after interpreter Thamsanqa Jantjie was exposed as an impostor.

"They resigned in December," ANC spokesman Keith Khoza told AFP.

He said he could not remember the date the couple left "but it was after the incident". He said they had given no reason for their departure.

The couple were directors at SA Interpreters, the company that employed Jantjie.

Mr Jantjie's performance sparked outrage among sign language experts who accused him of gesticulating gibberish at the Dec 10 memorial.

Mr Jantjie translated the eulogies - including those by United States President Barack Obama, United Nations chief Ban Ki Moon and Mandela's grandchildren.

His signing amounted to little more than "flapping his arms around", one expert said. He was "just making funny gestures", said another.

Mr Jantjie has claimed he is a qualified signer for the deaf but that his performance on the day was caused by a sudden episode of schizophrenia.

The South African government launched a probe into the incident shortly after the memorial, but six weeks on there has been no update on its progress.

The ANC has admitted Mr Jantjie previously worked for it at a party conference, but said it was unaware of any complaints regarding the quality of his services.

After the incident, the government formally apologised for any offence caused, and admitted a "mistake" had been made.