Joice Mujuru, once President Robert Mugabe's trusted deputy and his possible successor, has agreed to form a "pre-election alliance" with Movement for Democratic Change leader Morgan Tsvangirai.
The pair signed a memorandum of understanding at a public ceremony in Harare yesterday.
At the ceremony Mujuru, 62, said it had taken "more than six months" to come up with this agreement, which many hope marks the beginning of a strong challenge to Mugabe in polls due next year.
The MDC leader said it was the first step towards the formation of a "coalition government."
Mujuru said she would head the political committee while Tsvangirai would head a diplomatic committee.