ANC prevented its freed leader from confronting British PM over her refusal to back sanctions against South African apartheid

Nelson Mandela was "furious" when a top adviser stopped him meeting Margaret Thatcher two months after his historic release from prison.

A confidential US embassy cable says Mandela, visiting London in April 1990, was eager to spell out to Thatcher the recently unbanned African National Congress's objections to her policy on South Africa.

The Conservative prime minister had dismissed the ANC as "a typical terrorist organisation" and refused to back sanctions against the apartheid government, pursuing instead a policy of "constructive engagement". South Africa was then seen as a vital ally in stemming communist expansion.

Mandela's appointments secretary Zwelakhe Sisulu, son of the struggle veteran Walter Sisulu, persuaded the ANC to block the meeting, according to the memo dated 3 May 1990.

"Nelson Mandela is 'furious' with top adviser Zwelakhe Sisulu for persuading the ANC leadership to veto his plans to meet Margaret Thatcher in London in mid-April, according to a close Sisulu associate," it said.

"Gabu Tugwana (protect), Sisulu's deputy at the New Nation newspaper, told USIS [US propaganda agency] officer 27 April that Sisulu had been the most persuasive speaker at the ANC executive meeting which decided the issue of whether to meet Thatcher. We understand Mandela was keen for a Thatcher meeting but that Sisulu argued successfully against it."

The cable comments that the story "rings true". It explains: "Mandela has on several occasions expressed his eagerness for an early meeting with Thatcher to express the ANC's objections to her policy. We were consequently surprised when the meeting didn't materialise on his mid-April visit to London and suspected that ANC hardliners had nixed Mandela's plans."

The pair eventually met at 10 Downing Street in July that year.

A glimpse of the clamour and chaos around Mandela following the end of his 27-year prison stretch is offered by the document. It notes Sisulu, who had been seconded to work as Mandela's appointments secretary, had "drawn fire" from the ANC rank and file for his handling of the liberation hero's schedule. US embassy officials were frustrated by the "notoriously unreliable" way that Sisula managed Mandela's diary, the memo says, adding that it took seven weeks of "steady hounding" to get an appointment for the US ambassador with Mandela.

"In an ironic twist the formerly undependable Winnie [Mandela's wife] has now become one of the most effective channels to Mandela."

The cable concluded that the "overwhelming job" of keeping Mandela's appointment book was taking its toll on Sisulu. "'Mandela needs a government around him, not just me, to handle his schedule,' he recently lamented to us. With the still tight competition for influence in the Mandela entourage Sisulu's lack of administrative acumen could prove a distinct liability."

A cable from the US embassy in Pretoria on 17 December 1990 suggests that the huge optimism around Mandela's release was already fading. "A new mood of uncertainty has settled over South Africa following a year of unprecedented political change."

It says that after a promising start negotiations between the government and the ANC had become bogged down in mutual distrust. The ANC was suspicious that the government was trying to profit from escalating township violence; the government blamed the lack of movement towards constitutional negotiations on the ANC's lack of organisation and divisions inside the movement.

The memo warned: "If this deadlock persists beyond mid-1991 there is a real danger that this uneasy partnership will turn openly confrontational. The main reason for optimism is that neither the SAG [South African government] nor the ANC has any plausible long-term option other than negotiations. We still believe they will seize this option but the way ahead will be rough."

Mandela became president in 1994 after the country's first democratic, multi-racial election. Twelve years later David Cameron met Mandela and admitted the Conservatives had been wrong about the ANC.