Winnie Mandela's reported comments sharply contrast with a recent speech
South Africa's ruling ANC is asking Winnie Mandela to clarify comments attributed to her which starkly criticise her ex-husband, Nelson.
Mrs Mandela was quoted in UK newspaper the Evening Standard as saying former President Mr Mandela was a "sell-out" who had agreed a "bad deal for blacks".
The Mandelas were leaders of the struggle against the apartheid regime of the white-minority government.
The ANC said it wanted to verify the report before commenting further.
Mrs Mandela is thought to be in the US, and her office has refused to comment on the report.
She apparently made the comments in an interview with the Nadira Naipaul, wife of Nobel Prize winning author VS Naipaul.
The interview reportedly took place at Mrs Mandela's home in Soweto, Johannesburg.
The newspaper reported her as saying the name Mandela was "an albatross around the necks" of her family.
And she expressed disappointment that her former husband lost some of his revolutionary spirit after 27 years in jail.
The ANC has not commented on the substance of the article.
But party spokesman Ishmael Mnisi told the BBC: "As with any matter involving the ANC and its members, the ANC views this matter very seriously.
"We are interested to understand whether the report is true or not. If not, what we will want to know where it comes from."
The ruling party has been trying to reach Mrs Mandela since news of the report made local headlines on Tuesday afternoon.
"We are still waiting to hear from her, she has not gotten back to us," said Mr Mnisi.
Mrs Mandela is a senior ANC member and sits on the party's influential National Executive Committee (NEC).
The BBC's Pumza Fihlani in Johannesburg says the quotes attributed to Mrs Mandela are possibly the harshest public criticism so far levelled against the former president.
In the article, Mrs Mandela also indicated that her daughters - both in their 50s - now had difficulty accessing their father because of official red tape.
And the Standard reported that she said she "cannot forgive" her ex-husband for collecting the Nobel Peace Prize alongside FW de Klerk, the country's apartheid-era ruler.
"Hand in hand they went. Do you think de Klerk released him from the goodness of his heart? He had to," she was quoted as saying.
"The times dictated it, the world had changed, and our struggle was not a flash in the pan, it was bloody to say the least and we had given rivers of blood. I had kept it alive with every means at my disposal."
Correspondents say the comments stand in stark contrast to the warm speech she made a few weeks ago commemorating the 20th anniversary of Mr Mandela's release from jail.
On that occasion she praised his "unwavering commitment to the struggle".
Mr Mandela became South Africa's first democratic president in 1994.