Zimbabwe's ruling party and opposition are due to sign a deal outlining a framework for talks on the country's political crisis, both sides say, BBC reports.
Haile Menkerios, the UN's envoy to Zimbabwe, said the deal would be signed by President Robert Mugabe and opposition leader Morgan Tsvangirai.
South Africa's President Thabo Mbeki is to fly to Harare to witness the accord.
The two sides are locked in a dispute over presidential elections - which they both claim to have won.
The deal comes on the day that a new banknote is issued, for 100bn Zimbabwe dollars - the latest sign of the country's economic meltdown.
This is not quite enough to buy a loaf of bread and is worth less than US$1.
The agreement was due to be signed last week but Mr Tsvangirai pulled out.
Mr Tsvangirai garnered more votes in the initial presidential poll - but election officials said there was no outright winner and called for a run-off.
Mr Mugabe won the run-off - but he was the only candidate after Mr Tsvangirai withdrew, accusing the government of mounting a campaign of violence against his supporters.
"The signing will take place this afternoon," Justice Minister Patrick Chinamasa told the AFP news agency.
Opposition Movement for Democratic Change (MDC) officials also say they expect the agreement to be signed on Monday.
But the BBC's Jonah Fisher, in Johannesburg, says the opposition insists that the signing ceremony is not held at State House and should be low-key. It is due to be held in a five-star Harare hotel.
Our correspondent also notes that the five-page agreement to be signed does not go into the details of a possible power-sharing arrangement, which some see as the only way out of Zimbabwe's political and economic crisis - it is merely the starting point for substantive talks.
The document does not mention the central issue of Mr Mugabe's future.
Mr Mugabe insists that he must be recognised as Zimbabwe's president - a position rejected by the MDC, which accuses him of using violence to ensure that he won the run-off.
The MDC wants some kind of "transitional authority" to organise new, internationally-monitored elections.
Our correspondent says the deal is being seen as an important step forward, but only the first step of a difficult process.
