Margaret Thatcher dismissed Nelson Mandela as having “rather a closed mind” and expressed her disappointment after their first telephone conversation, according to secret files released at the National Archives.

The long buildup to the two leaders’ eventually successful meeting in July 1990 – five months after his release from a South African prison – is revealed in official prime ministerial records.

Mandela, then aged 71, was so eager to arrange face-to-face talks with the Conservative premier about sanctions that he telephoned Downing Street officials late at night requesting to see her the following morning.

The difference in expectations between the two politicians emerges in diplomatic cables and memos. Mandela, it has been suggested, was furious when one of his advisers supposedly persuaded the ANC to veto his initial plans to meet the British prime minister in April 1990 when he visited London.

The first note in the “secret” file entitled “Nelson Mandela’s visits to the UK” records the prime minister’s decision to “err … on the side of generosity” and invite him to talks and a working lunch on 4 July.

But on the evening of 16 June, Charles Powell, the prime minister’s foreign affairs adviser, was surprised to be called “out of the blue” at 11.45pm by Mandela, who was resting overnight “somewhere near Tunbridge Wells” on his way to Canada.

He was “very anxious” to see Thatcher about the easing of sanctions before he left the next morning for Heathrow. Would she be available at 8am? Powell thought not but offered to drive down to meet him or arrange a call. “He was rather insistent that he should speak directly to you,” he informed the prime minister.

Mandela and Thatcher did talk on the phone the next morning at 7.30am. He warned that relaxing sanctions too early could be counterproductive in the drive to end apartheid. She urged the ANC to abandon “armed struggle” and said the UK had “suffered at the hands of the IRA”.

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A four-page note documented their first direct exchanges. “The prime minister commented to me afterwards that she was a bit disappointed with Mandela, who seemed to have rather a closed mind,” Powell’s memorandum recorded.

“For his part, he will now have experienced first hand the prime minister’s strong views on the armed struggle and on sanctions, and this will no doubt influence his approach to the meeting on July 4th.”

The exchange does not appear to have been reported at the time and Powell noted: “We are not proposing to tell the press about this discussion.”

Sir Robin Renwick, the UK ambassador to South Africa, cabled lengthy, enthusiastic explanations of his discussions with Mandela to Downing Street which were clearly annotated by Thatcher ahead of their meeting and may have helped soften her stance.

“Mandela attaches a lot of importance to his meeting with the prime minister,” Renwick explained. “He is anxious to establish some kind of personal rapport (which should not be difficult given the character of the man).

“And to achieve a degree of support and understanding notwithstanding the political differences. He refers constantly to the prime minister’s meetings with Gorbachev [towards the end of the cold war] and clearly hopes to find himself being cast, on more direct acquaintance, as also the kind of person we can do business with.”

“He has suffered greatly for his cause,” Renwick added, “and, not surprisingly, has a burning sense of the injustices the black population have suffered.

“All those who visited Mandela in prison were struck by his courage and dignity … My own experience is the same. Mandela has a natural dignity and authority. He is not as intelligent as Mugabe but a great deal nicer.”

In a note to Thatcher shortly before she met the ANC leader in July, Powell said the aim should be “courteous straight-talking – of which Mandela will have heard regrettably little elsewhere, agreement to disagree on sanctions [the UK wanted to relax them early] but recognition by Mandela of your very considerable influence on events in South Africa and his wish to see you play a major part there”.

Renwick also briefed her. “Mandela shows his age,” he said. “His attention wanders during long statements. It is better to go for short exchanges and to keep drawing him out.”

Powell’s full record of the meeting suggests that the apprehension did evaporate. “It was an excellent meeting with an excellent atmosphere,” he wrote.

Mandela’s initial comments “lasted over 50 minutes uninterrupted: possibly a record”. He “implied that the [ANC] commitment to armed struggle could be given up quite soon”. He thanked her for the role she had played in securing his release from prison.

Thatcher told him there was absolutely no difference between them and that “apartheid must go”. South Africa was very fortunate, she said, to have President De Klerk and Mr Mandela at this juncture.