Election officials said the majority of the voters who took part in an early August election cast their ballots in favor of Robert Mugabe, in office since the 1980s.
British Minister for Africa Mark Simmonds said his government was frustrated with political freedoms in Zimbabwe.
"We believe it is essential that all allegations of electoral violations are thoroughly and independently investigated if a process is to be credible," he said in a statement Saturday.
Malawi hosts a weekend summit for members of the South African Development Community. Simmonds said the regional bloc should take a close look at the political environment in Zimbabwe during their meeting.
"I hope that SADC will look carefully at the electoral irregularities and continue to call for Zimbabwe Electoral Commission to release information, particularly the voters' roll," he said. "These irregularities seriously call into question the credibility of the elections."
Zimbabwean monitors said they observed some irregularities in terms of voter participation, though the African Union generally endorsed the elections.