Prime Minister Gordon Brown's spokesman said there should not be "undue delay" in announcing the outcome of Saturday's elections.

Official announcements of declarations in four separate sets of elections have began trickling out.

Noel Kututwa, chairman of the Zimbabwe Electoral Support Network, said the delay in declaring results was "fuelling speculation that something might be going on".

The elections for parliament, the presidency and local offices presented Mr Mugabe, 84, with the toughest challenge ever to his 28-year rule.

Mr Brown's spokesman said: "At the moment we are awaiting the formal announcement of the results of the elections in Zimbabwe. We very much hope that the announcement will not be delayed unduly.

"It's important, particularly for the people of Zimbabwe, that there is clarity about the situation regarding the elections as soon as possible."

Running against President Mugabe was chief opposition leader Morgan Tsvangirai, 55, who narrowly lost disputed 2002 elections, and former ruling party loyalist and Finance Minister Simba Makoni, 58.

If no presidential candidate wins more than 50% of the vote, there will be a run off.

Mr Tsvangirai's Movement for Democratic Change said he was leading the presidential race with 67% of votes, basing its unofficial count on returns from 35% of polling stations nationwide.