Cape Town — The scene has been set for a protracted legal battle between the South African Government and Gabriella Engels, the alleged victim of an assault by Zimbabwean First Lady Grace Mugabe.

If Engels - a Johannesburg model - wins her case, Mugabe may not be able to return to South Africa without facing prosecution.

Mugabe arrived back in Harare on Sunday after the visit to South Africa during which the assault is alleged to have taken place. She travelled with Zimbabwe's delegation to a summit of leaders of the Southern African Development Community (SADC).

Hours later, South African foreign minister Maite Nkoana-Mashabane announced that she had conferred diplomatic immunity on Mugabe, rendering her immune from prosecution. By making the announcement after Mugabe left the country, Nkoana-Mashabane averted any possibility of a court application to stop Mugabe from leaving while courts adjudicated on the validity of her decision.

Nkoana-Mashabane asserted that in reaching her decision she had taken into account the need to uphold both the law and Engels' rights. But she also cited “the imperative to maintain good inter-governmental relations” within the region and in particular with Zimbabwe. She suggested that spouses of heads of state enjoyed “derivative immunity”.

In response, the civil society group representing Engels claimed there were “very good grounds” to believe Engels would win a court challenge to the minister's decision.

Last week, a journalist with normally reliable contacts in the South Africa foreign ministry reported that the ministry's legal advisors believed Mugabe did not qualify for diplomatic immunity.

If Engels' lawyers overturn the minister's decision and government prosecutors fail to pursue the case, the lawyers will launch a private prosecution. Such a prospect would make it risky for Mugabe to return to South Africa.