The wife and children of Mark Thatcher, who is charged over an alleged coup plot, have landed in the UK on a flight from South Africa.
Diane Thatcher gave no comment on why she left South Africa
Texan-born Diane Thatcher, accompanied by Michael, 15, and Amanda, 11, landed at Heathrow Airport at around 0610 BST.
The family are reported to be en route to the US, where a spokesman said the children were to be enrolled in school.
Sir Mark is under house arrest after being accused of financing a planned coup in oil-rich Equatorial Guinea.
Emerging at the airport arrivals hall surrounded by police and airport staff, Diane Thatcher refused to comment on why she had left South Africa or on whether she would visit her mother-in-law - former UK Prime Minister Margaret Thatcher.
Baroness Thatcher returned to Britain from a holiday in the US last week after her son's arrest.
Sir Mark was arrested at his Cape Town home and released on a two million Rand (£165,000) bail bond but ordered to remain in the country ahead of a hearing in November.
He is accused of violating laws banning South African residents from taking part in foreign military action.
Sir Mark says he is innocent and he is co-operating with the authorities.
Police described finding packed suitcases at Sir Mark's home and accused him of planning to leave the country for good.
Family spokesman Lord Tim Bell said the children's American schooling had been planned some months ago and was nothing to do with the arrest.
Officials in Equatorial Guinea have asked South Africa for permission to interview Sir Mark.
The alleged plot leader, former British SAS captain Simon Mann, an old Etonian, has admitted trying to procure dangerous weapons.
He was found guilty of the charges by a Zimbabwean court on Friday but 66 other suspected mercenaries were acquitted.