Asked about Anglo American's reported $400m (£200m) investment in a Zimbabwe platinum mine, Mr Brown told parliament he did not want to see anything that would "prop up" the Mugabe regime.
The project, in the central district of Unki, would be the largest foreign investment in the country, the London Times reports.
Separately, the England and Wales Cricket Board said it had severed ties with the Zimbabwe Cricket team, cancelling a tour to England due for next year.
Police raid
Zimbabwe's presidential election run-off is due to go ahead on Friday, despite the opposition's withdrawal.
The leader of the Movement for Democratic Change (MDC), Morgan Tsvangirai, pulled out of the contest on Sunday, citing government-backed violence against his supporters.
Mr Tsvangirai, who is taking refuge in the Dutch embassy in Harare, says his party is open to suggestions from the emergency meeting of the Southern African Development Community (Sadc) peace and security committee being held in Swaziland.
The UK-based Guardian newspaper ran an article on Wednesday purportedly by Mr Tsvangirai, saying Mr Mbeki's "quiet diplomacy" had failed and calling for UN peacekeepers to go into Zimbabwe.
But MDC officials later contacted the BBC to disown the article, insisting neither Mr Tsvangirai or any other MDC member had written it - claims rejected by the paper.
Mr Mugabe, who blames the opposition for the violence, says he is open to discussions - but only after the vote, the Herald newspaper quoted him as saying.
Police raided an MDC building in the eastern city of Mutare on Wednesday, the AFP news agency reports, demanding ID cards and posting guards outside the premises.
Advisory role
The BBC's David Bamford says Sadc was assigned to oversee the election in Zimbabwe on behalf of Africa, and for that reason its opinion counts as to whether it believes Friday's election should go ahead if the opposition does not take part.
ZIMBABWE AND ITS NEIGHBOURS
Zimbabwe's opposition wants neighbouring countries to persuade
Robert Mugabe to step down. So how are relations changing?
South Africa's leader Thabo Mbeki remains the key mediator. He has not
criticised Mr Mugabe, despite pressure from the ruling ANC.
Zambian President Levy Mwanawasa called Zimbabwe a "regional
embarrassment", before suffering a stroke on 29 June.
Angola's President Jose Eduardo dos Santos is one of Robert Mugabe's closest allies. He has urged Mr Mugabe to end the violence.
Botswana said Zimbabwe's 27 June run-off vote was so flawed by violence
that it could not be considered legitimate.
Namibia is an ally of Robert Mugabe. It wants to re-distribute white-owned farms to black villagers. It has not criticised the violence.
Mozambique has hosted some white farmers forced out of Zimbabwe when their land was seized. It is seen as sympathetic to the opposition.
Tanzania's ruling party has a history of backing Robert Mugabe's
Zanu-PF party. Its foreign minister has condemned the violence.
DR Congo's President Joseph Kabila is an ally of Robert Mugabe who
sent troops to help his father, Laurent Kabila, fight rebels.
Malawi is seen as neutral. But 3m people from Malawi are in Zimbabwe
and many were badly hit by the farm invasions.
But that is not to say President Mugabe will necessarily pay any heed to its opinion, he adds.
The Swazi foreign minister, Mathendele Dlamini, told the BBC that the Sadc meeting was likely to offer advice to Mr Mugabe rather than issue any rebuke.
But the general secretary of the Confederation of South African Trade Unions (COSATU), Zwelinzima Vavi, said he hoped they would tell Mr Mugabe his presidency was over.
"The Sadc government must not drag themselves into recognising what everybody now agrees to be an illegitimate Robert Mugabe government," he said.
"We don't want Mugabe to be recognised at all, that should be the starting point."
Kenya's leaders have also joined international condemnation of Mr Mugabe and his government.
Prime Minister Raila Odinga said Mr Mugabe had no right to call himself president and said Friday's vote would have no legitimacy and should be postponed.
"He lost an election and if he now proceeds to go and conduct a sham election and declare himself as a president that is not going to be acceptable," he said.
Unified effort
The US has said it would not recognise the result of any vote held on Friday because of the violence being waged against the opposition.
The MDC says some 86 of its supporters have been killed and 200,000 forced from their homes by militias loyal to the ruling Zanu-PF party. The government blames the MDC for the violence.
The Southern African Catholic Bishops' Conference issued a statement on Wednesday saying "the politically-motivated violence, intimidation and torture have made a just and fair run-off presidential election virtually impossible".
They urged a unified effort from the international community and southern African countries to help resolve the situation and avoid a "vast humanitarian crisis that will engulf the whole Southern African region".
The MDC won the parliamentary vote in March, and claims to have won the first round of the presidential contest - held on the same day - outright.
According to official results, Mr Tsvangirai was ahead of Mr Mugabe but failed to gain enough votes to avoid a run-off.
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