South Africa has granted diplomatic immunity to Zimbabwean First Lady Grace Mugabe, allowing her to leave the country without answering questions about an assault allegation.

President Robert Mugabe and his wife arrived back in Harare early on Sunday.

Mrs Mugabe, 52, is accused of assaulting model Gabriella Engels a week ago at a Johannesburg hotel where her sons were staying.

She has not commented publicly on the case.

A lawyer for Ms Engels said that they were planning to challenge the immunity decision in court.

Willie Spies said he would argue that the first lady should not have been given special treatment as she had been in South Africa on private business.

The South African government’s decision came in a notice from International Relations Minister Maite Nkoana-Mashabane, published in the Government Gazette on Sunday.

“I hereby recognise the immunities and privileges of the First Lady of the Republic of Zimbabwe, Dr Grace Mugabe,” she said.