Prime Minister Gordon Brown said that he hopes African leaders meeting Zimbabwean president Robert Mugabe "will make it absolutely clear that there has got to be change" in his country.
He said: "I think the message that is coming from the whole world is that the so-called elections will not be recognised. People want the violence and intimidation to end."
As the African Union met Mr Mugabe in Egypt, Mr Brown said: "A new government has got to be brought in."
He added that when that happened, the UK would be ready to help the country to rebuild. He was speaking to reporters during a visit to launch the NHS constitution.
Mr Mugabe is at the African Union summit which opened in the Red Sea resort of Sharm el-Sheikh in Egypt. African leaders are expected to use the two-day meeting to discuss the political crisis in Zimbabwe.
The 84-year-old was declared the landslide winner of an internationally-condemned poll, marred by violent intimidation, in which he was the sole candidate. He was sworn in for a sixth term on Sunday night.
But some African leaders have been unwilling to criticise Mr Mugabe and have called for the president and opposition to engage in dialogue.
African observers in Zimbabwe called for a re-run, saying the election was not free and fair and supporters of Mr Mugabe were beating people who could not prove they voted.
Movement for Democratic Change (MDC) leader Morgan Tsvangirai pulled out last week in protest at a brutal government-organised campaign against his supporters.
But a defiant Mr Mugabe went ahead with the swearing-in ceremony. State media in Harare, however, did report that he was prepared to hold talks with the MDC.
