Britain said the result of the Zimbabwe election was not credible without an independent investigation of alleged vote-rigging, as Robert Mugabe prepared to be be sworn in for another term as president on Thursday.

Opposition leader Morgan Tsvangirai launched a legal challenge against the result, alleging up to a million eligible voters were excluded and demanding that electoral rolls be released.

He condemned the election as "a farce", but in a surprise U-turn last week, Tsvangirai and his Movement for Democratic Change (MDC) withdrew the petition, saying he would not get a fair hearing.

British Foreign Secretary William Hague said: "I am extremely concerned that the MDC-T had to withdraw its legal challenge due to concerns over the independence of the judiciary.

"I strongly believe that an independent investigation of any allegations of election irregularities would be required for the election result to be deemed credible."

Hague reiterated that he had "grave concerns" over the conduct of the election and the "flaws" highlighted in the initial assessments of the the South African Development Community (SADC) and African Union (AU) observation missions.

"There is strong evidence that these elections fell short of SADC's own guidelines and the Zimbabwean electoral law," he said in a statement.

"We are concerned about the potential implications for the region."

Mugabe, 89, will usher in a new five-year term in a 60,000-capacity stadium in a ceremony to which around 40 leaders have been invited.

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