Sir Mark Thatcher is facing a legal battle to avoid jail after being arrested and charged in South Africa with involvement in a failed coup attempt in the oil-rich west African state of Equatorial Guinea.

Lady Thatcher's son was arrested at his suburban Cape Town mansion whilst still in his pyjamas at 7am by an elite police squad known as the Scorpions.

Police said they had "credible evidence" that he was involved in backing an attempt to overthrow the president of the west African state.

His appearance at a nearby magistrates court was then delayed, apparently because Sir Mark was robbed in his holding cell by other prisoners who, according to a court official, stole his mobile phone, shoes and jacket. All belongings were later returned.

Last night, he was confined to his home in the upmarket Cape Town suburb of Constantia after being released on strict bail conditions which require him to remain under virtual house arrest until he has posted bail of two million rand (£165,000).

He was also ordered to surrender his passport and stay in the Cape Town area until another court appearance on November 25.

In a statement after his release, he denied any involvement in the coup plot.

Writing in the Telegraph, Charles Moore, the authorised biographer of Baroness Thatcher, concludes: "His only fault in relation to her - and her's in relation to him - is an excess of zeal over judgment. If he has to spend her last years in a South African jail, her sense of sadness and loss will be immense."