In a briefing in Pretoria, Zimbabwe's ambassador to South Africa presented what he said was a letter from Brown to Britain's Law Society in which he is said to promise to continue funding Zimbabwean groups working for "democratic change".

"Clearly such effort is meant to fuel tensions towards the March 29 plebiscite in Zimbabwe," Ambassador Simon Khaya Moyo told South Africa's Institute of Security Studies, an independent think-tank.

Law Society spokesman Steve Rudaini confirmed Brown had written to Andrew Holroyd, the group's president, concerning the situation in Zimbabwe. Rudaini said the letter was no longer available and had been previously published in error.

Zimbabwean President Robert Mugabe frequently accuses Western powers, especially former colonial ruler Britain, of working with the Movement for Democratic Change (MDC), the main opposition party, and others to oust his government.

Moyo on Tuesday said Britain and the United States were trying to bring about "regime change" in Zimbabwe and warned that Zimbabweans would not tolerate interference in their internal affairs.

The British and U.S. governments have been sharply critical of Mugabe, accusing him of widespread human rights abuses, stifling dissent and destroying the economy of the once-prosperous nation during his 28-year rule.

They and other Western nations have imposed sanctions on Mugabe and his top officials.

The 84-year-old Zimbabwean leader is running for another five-year term in the elections next month. He has vowed to crush his rivals, including former finance minister Simba Makoni and MDC leader Morgan Tsvangirai.

Makoni is running as an independent and claims to have the backing of unidentified officials in Mugabe's ruling ZANU-PF party. Makoni was formally expelled from ZANU-PF earlier this month for challenging Mugabe.

Both Makoni and Tsvangirai have promised to rescue Zimbabwe's economy, which has been devastated by annual inflation of over 100,000 percent, unemployment of more than 80 percent and chronic food and fuel shortages.

Moyo said Mugabe's government would not context the results of the election if the MDC won the polls.