The rape allegation against Cosatu general secretary Zwelinzima Vavi is politically motivated, the National Union of Metalworkers of SA's (Numsa) Kwazulu-Natal branch said on Sunday.

"This latest rape allegation cannot be de-linked from the poisonous, malicious and well-calculated political offensive directed towards destroying comrade Vavi's political integrity and standing in society," Numsa regional secretary Mbuso Ngubane said in a statement.

He said the union was "fully behind" Vavi.

"The presumption of innocence until proven otherwise... should be upheld by all, as opposed to the gutter of public opinion."

Meanwhile, the Azanian People's Organisation said it was "disturbed" by the rape allegation.

"We hope this allegation would not stick on him," said Azapo spokesman Funani ka Ntontela.

"We hope this allegation is not engineered within the tripartite alliance as a weapon to weaken Vavi in his stand against graft and maladministration in this country."

However, if Vavi was charged, Azapo would encourage him to step down from public representation, Ntontela said.

The Sunday Times published an open letter from a woman accusing Vavi of raping her.

In it, she claimed Vavi went to her office last January last year.

"He is a big man. When I got up, he grabbed me and then kissed me. Then he locked the door. I still have the dress I wore on that day, " she said without giving more details.

On Saturday, Vavi released documents to the media, including an internal complaint by the woman in which she accused him of rape.

Vavi admitted to having an extramarital affair with her, and claimed she had later tried to extort R2 million from him.

He denied raping her, and said the sex was consensual.

The police could not confirm that a charge of rape had been laid against him.