The drive by the Zimbabwean people to rid themselves of tyrannical rule has reached a "critical moment", Foreign Secretary David Miliband said.
Mr Miliband said Robert Mugabe and his "thugs" had made an election impossible.
His comments followed the decision by opposition MDC leader Morgan Tsvangirai to pull out of Friday's presidential run-off.
The Foreign Secretary said: "We have reached an absolutely critical moment in the drive by the people of Zimbabwe to rid themselves of the tyrannical rule of Robert Mugabe.
"He has made and his thugs have made an election impossible and so now we face a critical crisis of legitimacy because it's clear that the only people with any shred of legitimacy are the people who won the March 29 first round and that was the opposition."
Speaking at a news conference in the capital Harare earlier, a tired-looking Mr Tsvangirai told reporters a credible election was "impossible".
"We can't ask the people to cast their vote on June 27 when that vote will cost their lives," he said. "We will no longer participate in this violent sham of an election. We will not play his game."
Earlier, in the latest move to disrupt the opposition, thousands of ruling party militants blocked the site of a planned MDC rally.
Shadow foreign secretary William Hague said: "The MDC's withdrawal from the election is wholly understandable in the face of the rigging, violence, and murder perpetrated by the Mugabe regime.
"It is now clear beyond doubt that Zimbabwe is suffering under one of the world's vilest and most despotic tyrannies."
