Prime Minister Gordon Brown said on Saturday he was appalled by signs Zimbabwe's government was resorting to violence in an election standoff and warned President Robert Mugabe the world was losing patience.
Human rights organisations and the opposition Movement for Democratic Change (MDC) say Mugabe has unleashed a campaign of violence after the ruling ZANU-PF party lost control of parliament in a March 29 election.
Official results of a presidential vote - which the MDC says its leader Morgan Tsvangirai won - have not been announced.
"I cannot understand why it is taking so long to announce the result of the presidential elections," Brown said in a statement, his toughest so far on the Zimbabwe situation.
"I am appalled by the signs that the regime is once again resorting to intimidation and violence," he said.
"We will be vigilant. The international community will remain careful to do nothing to undermine efforts to secure an outcome that reflects the democratic will of the people of Zimbabwe," he said.
"But the international community's patience with the regime is wearing thin," he added.
Britain, the former colonial power, has often clashed with Mugabe, who has ruled Zimbabwe since independence 28 years ago.
Since the election, the government has measured its words carefully for fear Mugabe could portray its comments as interference in the country's politics, analysts say.
Officials say the onus is on Zimbabwe's neighbours to take the lead in resolving the political and economic crisis in the country.
"The Zimbabwean people have demonstrated their commitment to democracy. We, and the leaders of the region, strongly share this commitment," Brown said.
Zimbabwe's opposition called a general strike on Friday after officials said Mugabe would snub a regional summit called to discuss rising fears of bloodshed over the delayed election results.
As tension increased, police accused the MDC of "spoiling for a fight" and of deploying 350 youth wing members around the country. The police banned a Sunday rally by the MDC.
