Margaret Thatcher approved of a failed attempt to use an army of mercenaries to overthrow the president of Equatorial Guinea, according to the unpublished memoirs of the chief protagonist of the bid — former SAS officer Simon Mann.

The former prime minister, whose son, Mark, was convicted in a South African court of involvement in the attempted 2004 coup, allegedly told Mann at a meeting at her Belgravia home, "I'm sure it's going to work."

It is alleged that Thatcher likened the need for radical change in oil-rich Equatorial Guinea to the way London's Docklands had been redeveloped during the 1980s. She is also said to have encouraged Mann to talk to a group seeking to overthrow Venezuela's then-president, Hugo Chavez, with the words, "We must always look after our friends, Simon . . . as I'm sure you know."