Gordon Brown attacked Zimbabwe's sham election as "a new low" for the Mugabe regime and urged African countries to help ensure the "sickening chapter" was closed.
Votes were being counted on Saturday after a presidential run-off, marked by reports of widespread violent intimidation, in which Mr Mugabe was effectively the sole candidate.
Although opposition leader Morgan Tsvangirai's name appeared on the ballot, he withdrew last weekend in protest at a brutal campaign against his Movement for Democratic Change supporters, .
Mr Brown said: "The world is uniting in rejecting the illegitimate regime of Robert Mugabe.
"We will work with international partners to find a way to close this sickening chapter that has cost so many lives.
"The forthcoming African Union summit is an opportunity for the region to restore hope to the people of Zimbabwe. Democracy will ultimately prevail."
He cited former South Africa president Nelson Mandela, who spoke out this week while in the UK for a celebration on Friday night of his 90th birthday.
"As Nelson Mandela has said, there has been a failure of leadership in Zimbabwe. Yesterday's attempt to hold an election was a new low."
The United Nations Security Council issued a statement on Friday night deeply regretting the election but stopped short, reportedly under pressure from South Africa, from declaring it illegitimate.
