CAPE TOWN, Jan 13: The son of former British Prime Minister Margaret Thatcher pleaded guilty on Thursday to a role in a foiled mercenary plot in west Africa under a plea bargain to avoid prison.

The Cape High Court agreed to a deal for Mark Thatcher to pay a fine of three million rand ($500,000) or face five years in jail in South Africa, in addition to a further 4-year prison sentence suspended for five years.

Prosecutors said Thatcher was free to leave South Africa and he left his Cape Town home for the airport on Thursday evening and was expected to rejoin his family in the United States.

His American-born wife Diane took their two children to the United States after his arrest, but returned to Cape Town to be with him. Margaret Thatcher visited her son at Christmas.

When asked in court whether he agreed to the various terms of the deal, a nervous-looking Thatcher dressed in a suit and gold-coloured tie said "Yes I do, Yes I do" repeatedly.

Thatcher agreed to assist South African investigations into the plot against the government of Equatorial Guinea, a tiny country flush with newly-found oil wealth. "There is no price too high for me to pay to be reunited with my family and I am sure all of you who are husbands and fathers would agree with that," Thatcher said on the steps of the court after the hearing.

A mocking banner strung from the third storey of an office block opposite the courthouse read "Save me mummy". It was not clear who placed it. On the steps outside the court one man chanted "Shame on you, shame on you".

ANGRY REACTION: The plea bargain drew an angry reaction from the youth wing of South Africa's ruling African National Congress. "This is indeed an abomination and miscarriage of justice," a league statement said, adding that Thatcher had got away with "nothing more than a slap in the wrist".