The accusations were made despite the clearest signs yet that the two sides could soon sign a power-sharing deal to end political turmoil that has worsened Zimbabwe’s economic decline.
"In respect of these talks, they are directly taking orders and advice from British and American diplomats here, orders to get a deal that strips President Mugabe of all power and makes him a ceremonial head of state."
The negotiations started after Mugabe’s re-election unopposed in a June presidential poll that was boycotted by Movement for Democratic Change (MDC) leader Morgan Tsvangirai and condemned around the world.
Mugabe said on Wednesday he was hopeful that a power-sharing deal could be signed on Thursday.
Tsvangirai also sounded an optimistic note, saying he had been told by South African President Thabo Mbeki that "very little work" was needed to finalise the deal.
MDC spokesman Nelson Chamisa dismissed the ZANU-PF charges of collusion with the Western countries, which have imposed sanctions on Mugabe and his inner circle to try to force change.
"I am not going to get into details of the talks except to say we are negotiating in good faith and with an open mind. You cannot say the same about ZANU-PF."
Despite the statements from both sides this week, many Zimbabweans question the optimism.
Tsvangirai, who won the initial election in March but failed to get adequate ballots for an outright victory, boycotted the second vote over attacks on his supporters.
Zimbabweans hope a deal can pave the way for the re-habilitate an economy in meltdown, as shown by inflation of over 11 million percent and severe food and fuel shortages that have driven millions across the country’s borders, straining regional economies. Reuters