"Mandela will die soon. Today, tomorrow, this week, next week. It wont be long. Remember this, he out-lived Thatcher. When Cameron latches on the Mandela bandwagon this week remember that in 1985 he was a top member of the Federation of Conservative Students, who produced the "hang Mandela" posters. (This is one of the posters)

In 1989 Cameron worked in the Tory Policy Unit at Central Office and went on a anti-sanctions fact finding mission to South Africa with pro-apartheid Lobby Firm that was sponsored by Botha. Remember this when he tells the world he was inspired by Madiba."

David Cameron accepted an all-expenses paid trip to apartheid South Africa while Nelson Mandela was still in prison, an updated biography of the Tory leader reveals today.

The trip by Mr Cameron in 1989, when he was a rising star of the Conservative Research Department, was a chance for him to "see for himself" and was funded by a firm that lobbied against the imposition of sanctions on the apartheid regime.

Critics described it as a "sanctions-busting jolly" that raised questions about the character of the man who, after a week when the Government's credibility on the economy hit a new low, is now on course to be prime minister in a little more than a year's time.

Mr Cameron will portray himself as prime minister-in-waiting today when he addresses his party's spring conference in Cheltenham with a promise to introduce a "government of thrift".

The trip is revealed for the first time in a newly updated edition of Cameron: The Rise of the New Conservative, by James Hanning, the deputy editor of The Independent on Sunday, and Francis Elliott, the deputy political editor of The Times.

Mr Cameron's office insisted the visit by the 23-year-old future leader was a "fact-finding mission" that took place 20 years ago, and the Thatcher government was opposed to sanctions against South Africa at the time.

He met union leaders and black opposition politicians, including the head of the left-wing Pan Africanist Congress (PAC) during the trip, a spokesman said. The trip was organised and funded by Strategy Network International (SNI), created in 1985 specifically to lobby against the imposition of sanctions on South Africa.

Yet when asked by the authors if Mr Cameron wrote a memo or had to report back to the office about his trip, Alistair Cooke – in 1989 his boss at Central Office – said it was "simply a jolly", adding: "It was all terribly relaxed, just a little treat, a perk of the job. The Botha regime was attempting to make itself look less horrible, but I don't regard it as having been of the faintest political consequence."

In 2006, shortly after becoming Tory leader, Mr Cameron tried to make amends for his party's record on South Africa by visiting Nelson Mandela, by then President, at the same time as suggesting the party's policy from the 1980s was from another era that he did not belong to. He said at the time: "The mistakes my party made in the past with respect to relations with the African National Congress and sanctions on South Africa make it all the more important to listen now.

"The fact that there is so much to celebrate in the new South Africa is not in spite of Mandela and the ANC; it is because of them – and we Conservatives should say so clearly today."

But what was not known, and which sheds new light on the 2006 meeting with Mr Mandela, was that Mr Cameron himself paid an official visit to the country under the apartheid regime. Civil servants and advisers were told not to go on such trips, the book reveals.

Peter Hain, the former Cabinet minister and prominent anti-apartheid campaigner, said last night: "David Cameron asks us to judge a leader's character – well, Gordon Brown at this time was active in the anti-apartheid movement, while Cameron was enjoying a sanctions-busting jolly. That is a measure of character.

"This just exposes his hypocrisy because he has tried to present himself as a progressive Conservative, but just on the eve of the apartheid downfall, and Nelson Mandela's release from prison, when negotiations were taking place about a transfer of power, here he was being wined and dined on a sanctions-busting visit.

"This is the real Conservative Party, shown by the fact that his colleagues who used to wear 'Hang Nelson Mandela' badges at university are now sitting on the benches around him. Their leader at the time Margaret Thatcher described Mandela as a terrorist."

A spokeswoman for Mr Cameron said: "Yes, he did go to South Africa. He met with anti-apartheid campaigners, he met opposition politicians when he was out there, including Zeth Mothopeng, the head of the PAC.

"It was a fact-finding mission that happened 20 years ago. He met union leaders and was shown around mines. The position of the Conservative Party at that time was against sanctions."

The trip was offered to the Conservative Research Department by Derek Laud, who was employed by SNI and was later a Big Brother contestant.