The head of the United Nations offered Robert Mugabe a lucrative retirement package in an overseas haven if he stood down as Zimbabwe's president, according to claims quoted in leaked diplomatic cables.

The extraordinary offer was allegedly made by Kofi Annan, who was then the UN secretary general, at the millennium summit of world leaders in New York, according to a memo drawn up by American officials which was obtained by the WikiLeaks website.

The memo, written in September 2000, records a meeting between a US embassy official in Harare and a senior source in the Movement for Democratic Change (MDC), the party opposed to Mugabe's Zanu-PF.

According to the MDC source, whose name the Observer has redacted, "Kofi Annan, in the recent meeting in New York during the millennium summit offered Mugabe a deal to step down. Although [the MDC source] said the MDC was not privy to the details, he surmised that Annan's supposed deal probably included provision of safe haven and a financial package from Libyan president [Gaddafi]. The opposition party heard that Mugabe turned down the offer the following day after discussing it with the first lady."

The offer, which many Zimbabwean experts may simply dismiss as wishful thinking on the part of a frustrated MDC, was not the only one rumoured to have been made to Mugabe at that time. The cable reveals that Zanu-PF itself had put out "feelers" to see whether the MDC would be willing to allow Mugabe a "graceful exit" that was "in Zimbabwe's national interest".

The MDC source said the business interests of senior Zanu-PF members were being badly damaged by the "current economic and political situation. They blame President Mugabe and are determined to find a way to ease him out in a dignified way."

The cable notes that the MDC "is gaining strength in the rural areas" and that the MDC president, Morgan Tsvangirai, "has agreed that it is in Zimbabwe's best interests for the MDC to do all it can to secure a graceful exit strategy that preserves somewhat of a positive legacy for Mugabe. Otherwise the president would have little incentive to go."

The memo also contains the claim that an international arms dealer once reputed to be a key Mugabe ally worked for British intelligence.

A Zanu-PF source is quoted in the cable suggesting that "John Bredencamp [sic], a shady white Zimbabwean businessman, had told Zanu-PF he would provide a financial 'retirement' package for Mugabe".

The cable explains that the source "did not know whether Bredencamp had sufficient resources to make such a package attractive enough, but he claimed that Bredencamp worked for MI6 and could be a channel for the British to provide funds to sweeten the deal". The cable goes on to note that the British high commission in Harare "scoffed at the very idea".

The references to Bredenkamp, a former Zimbabwean rugby captain, are intriguing. The multimillionaire, who has a home in Berkshire, has rejected claims that he is a Mugabe crony. Bredenkamp, who made his money in tobacco farming, was named in a 2002 UN report as a key arms trader who made millions of pounds from illegally exploiting natural resources in the Democratic Republic of Congo. Bredenkamp, who did not respond to Observer emails, has rejected the UN's claims and has pursued legal action to clear his name.

Rumours that Bredenkamp and his companies have worked with British intelligence have been rife in Zimbabwe for two decades.

Some have suggested that it would have been inconceivable for Bredenkamp to operate his business empire without a close relationship with MI6, but Bredenkamp has not commented on these claims and is likely to dismiss them.

The cable also reveals that the US was pressed to play the part of an "honest broker", organising a conference to address land allocation and amnesties in a post-Mugabe nation because of concerns that Britain was not suitable for the role.

According to a prominent banker in Zimbabwe, who acted as a go-between for the Zanu-PF, the broker would have to "underwrite the costs of whatever agreement emerged. The British government, he claimed, has 36 million pounds available for land reform in Zimbabwe, but they are probably too antagonistic to play an honest broker role. The Americans, though, probably would be acceptable."

The issue of providing settlements for senior officials in the Mugabe regime was addressed last year when the US and the UK were asked to pay into a trust fund that would ensure Zimbabwean military officials enjoy a comfortable retirement.

A separate memo, written in October last year, reveals that Elton Mangoma, the minister of economic development and member of Morgan Tsvangirai's inner circle, had asked the US to contribute to a "trust fund" to buy off the "securocrats".

The memo notes: "Mangoma said that a primary obstacle to political progress and reform was the service chiefs. Unlike many ZANU-PF insiders who had stolen and invested wisely, these individuals had not become wealthy. They feared economic pressures, as well as prosecution for their misdeeds, should political change result in their being forced from office. Therefore, they were resisting…progress that could ultimately result in fair elections. Mangoma asked for consideration of US contribution to a 'trust fund' that could be used to negotiate the service chiefs' retirement." He said he planned to approach the UK and Germany with the same request."