Dickens & Madson gave all information to the Government of Zimbabwe

Company statement

The plot was allegedly uncovered with the release of grainy video footage, purporting to show opposition leader Morgan Tsvangirai discussing Mr Mugabe's "elimination".

Mr Tsvangirai has vigorously dismissed the charges as a crude smear campaign designed to discredit him ahead of Zimbabwe's upcoming presidential election.

But political consultancy Dickens & Madson, which says it was approached first by Mr Tsvangirai, said it felt it had to turn evidence of the plot over to the Zimbabwean authorities.

Company President Ari Ben Menashe, reportedly a former Israeli intelligence agent with long-standing links to the Mugabe government, has denied the video was part of a sting operation aimed at discrediting Mr Tsvangirai.

Controversial figure

The footage, allegedly filmed in Montreal last December, is of such poor quality that it is impossible to identify anyone beyond doubt.

Australia's SBS Dateline programme said it showed a meeting between representatives of the political consultancy and a man described in the film as "the MDC's top man".

The MDC says Mr Mugabe's party is trying to discredit Mr Tsvangirai

In it, the figure said to be Mr Tsvangirai describes the procedure in the country immediately after "the head of state has been eliminated".

Mr Ben Menashe, a controversial figure linked in the 1980s to the Iran-Contra arms scandal, has said he believes Mr Tsvangirai did not realise that he had already worked as a lobbyist for Mr Mugabe.

"So he knocked on the wrong door," Mr Ben Menashe told Britain's Daily Telegraph newspaper.

In a statement, the company said Mr Tsvangirai made an approach through an intermediary, "knowing of the extensive contacts Dickens & Madson principals have with various intelligence agencies around the world".

But once he had filmed proof of what Mr Tsvangirai was allegedly asking for, he went to the Zimbabwean authorities.

"Dickens & Madson felt itself morally compelled to assist the embattled people of Zimbabwe and their President Robert Mugabe," the statement said.

Although Zimbabwe's government-owned Herald newspaper reported the story on its front page, no legal action has so far been taken against Mr Tsvangirai.

Assassination talk

Mr Tsvangirai's MDC party said in a statement that the assassination story was similar to numerous other baseless stories put out at the instigation of the ruling Zanu-PF party.

MDC chief spokesman Welshman Ncube, said the meeting did take place, but only after the company had approached Mr Tsvangirai, calling for an urgent meeting to discuss policy issues.

Mr Ncube confirmed that two meetings had taken place in London before the Montreal meeting, which was filmed.

But talk of assassination was entirely initiated by Mr Ben Menashe, he said.

News reports have made much of Mr Ben Menashe's colourful past as a notorious figure in the world of espionage and arms dealing.

He was arrested in 1989 for allegedly selling aircraft to Iran, but was later acquitted.

He has also accused the Israeli intelligence agency, Mossad, of murdering press tycoon Robert Maxwell.