Agang SA leader Mamphela Ramphele has proven she cannot be trusted to see a project to the conclusion, DA leader Helen Zille said on Sunday.
"We have been through many false starts," she said in a statement.
"When Dr Ramphele insisted on Monday that we go public on Tuesday to announce her acceptance of our offer of the DA's presidential candidacy, we accepted that she had finally made up her mind."
Zille said Ramphele had reneged on the agreement to be the Democratic Alliance's presidential candidate at a meeting on Sunday despite the party negotiating in good faith.
Confusion arose on Friday over Ramphele's joining the DA.
According to a joint statement issued by Ramphele and Zille on Friday, the Agang SA founder would be welcomed into the DA at a press conference in Johannesburg on Monday.
However, following the joint statement being issued, a message from Ramphele was uploaded onto Agang SA's website.
"You may have by now seen 'joint' statements issued by the Democratic Alliance in which it is claimed that I will be accepting DA membership on Monday," she said.
"This is not true. Nor did I agree to any such statement. It is Agang SA's position that the technical committee must first complete its work. I am leader of Agang SA, and Agang SA will continue its work to restore the promise of our freedom."
Zille on Sunday said Ramphele was playing a game of "cat and mouse" telling the media one thing, Agang supporters another thing and the DA another.
"It is not clear what her objective is, but whatever it is, it is not in the interests of the South African people," she said.
"By going back on the deal... just five days after it was announced, Dr Ramphele has demonstrated -- once and for all -- that she cannot be trusted to see any project through to its conclusion."