JOHANNESBURG (AFP) - Mr Nelson Mandela's wife Graca Machel called off a trip to London this week to be with her ailing husband, who was readmitted to hospital with a lung infection on Saturday.

"Mrs Graca Machel cancelled her appointment in London on Thursday," South African presidency spokesman Mac Maharaj told Agence France-Presse.

"This morning when Madiba was hospitalised, she accompanied him, and she is with him at the hospital," he said, using Mr Mandela's clan name.

Ms Machel was scheduled to attend a summit on hunger in London hosted by British Prime Minister David Cameron, which opened on Saturday.

South Africa's revered first black president and Nobel Peace Prize winner is in a "serious but stable" condition in a Pretoria hospital, his fourth hospital stay in seven months.