Favourite child's troubles have upset doting mother

Friends of Baroness Thatcher said yesterday that she would be deeply upset about the allegations made against her son, Sir Mark, and are worried about the impact they will have on the 78-year-old former prime minister.

Lady Thatcher, who is due to return to Britain today from a holiday in the US, is close to her son. Although her late husband, Sir Denis Thatcher, is said to have taken a dim view of some of Sir Mark's business activities, friends said that she had always found it hard to believe that he could do wrong.

Charles Moore, her official biographer and a former editor of The Daily Telegraph , said: "If he has to spend her last years in a South African jail, her sense of sadness and loss will be immense." He told BBC Radio 4: "She is a fairly old and fairly frail lady, and she is also since last year a widow. One of the things that was helping her old age was the capacity to rest more and to stay with Mark in South Africa and to see her grandchildren."

As Prime Minister, Lady Thatcher defended her son's business dealings when he appeared to trade on her name, insisting he was "batting for Britain" when he received commission on a contract she signed in Oman. Aides feared that she would be embroiled in a major scandal if he carried on such activities and he was encouraged to go to live in America.