Foreign Secretary Jack Straw has criticised last week's elections in Zimbabwe as "seriously flawed".
Mr Straw said it was inevitable that Mr Mugabe's government would fall
In the Commons, Mr Straw condemned President Robert Mugabe's exclusion of global media and independent observers and said electoral abuse was "rife".
The ruling Zanu-PF won a two-thirds majority, which Mr Straw said did not reflect Zimbabweans' "democratic will".
Britain would continue an arms embargo and sanctions in protest, but would not take military action, Mr Straw said.
"The United Kingdom will continue to work with its international partners for a return to an accountable, democratic government which respects the rule of law and human rights of Zimbabweans," he said.
Mr Straw said Britain had given more than £71m in food aid since September 2001 and £26.5m towards efforts by non-government organisations to tackle Aids.
The declared results of the 31 March election gave Mr Mugabe's Zanu-PF party 78 seats, the opposition Movement for Democratic Change (MDC) 41 and an independent candidate one seat.
Mr Mugabe can give 30 seats by appointment, giving him the two-thirds majority required for amending the constitution.
Mr Straw said: "These are the official published results. There is however, strong evidence that they do not reflect the free, democratic will of the Zimbabwean people."
Some people were said to have been prevented from voting
He acknowledged there was less violence than previous elections, but said "overall the election process was seriously flawed".
"Thousands were turned away from the polling booths, there are serious, unexplained discrepancies between votes tallied and the official numbers later announced.
"Other abuse was rife. This included food aid being misused, ghost voters, a lack of equal access to the media, abuse of draconian security legislation and an election commission packed with Zanu-PF supporters," he added.
He said even observers approved by Mr Mugabe had commented that at least 10% of voters were prevented from casting their vote, while independent observers put the figure closer to 30%.
Southern African observers have endorsed the result.
The Southern African Development Community said the poll was "peaceful, transparent, credible and well managed", although they expressed concern at the opposition's lack of access to state-owned media.
Mr Straw said: "I'm surprised and saddened that Zimbabwe's neighbours have chosen to ignore the obvious and serious flaws in these elections and have declared them fair."
Conservative Henry Bellingham said the election was "a total insult to democracy" and asked whether pressure could be put on South Africa to take a stronger stance.
The Foreign Secretary said the South African government's position towards Zimbabwe was "regrettable" but that Britain and the EU believed the best way forward was to "maintain a very close dialogue" with its government.
He added that if President Mugabe's government fell the responsibility for rebuilding Zimbabwe would fall on everybody in the international community, including South Africa.
"I cannot say how long President Mugabe will stay.
"What I know for certain is that he lacks any effective consent of the people, that beneath this apparently strong veneer this is a weak and fragile government which will collapse sooner rather than later," he said.
Liberal Democrat Sir Menzies Campbell asked: "Just how revealing do you think it was when Mr Mugabe thanked the people of Zimbabwe for having voted, in his words, correctly?"
Mr Straw replied: "That one adverb spoke volumes, because there was huge intimidation of voters."