Gordon Brown has urged the world to tell Zimbabwean President Robert Mugabe "enough is enough" amid growing concern over the country's cholera outbreak, reports BBC.
The prime minister said the crisis, which has claimed nearly 600 lives, had become an international emergency.
Government in Zimbabwe was now broken and the state was unable or unwilling to protect its people, he said.
Mr Brown added that medicine would get to those who needed it, regardless of any differences with Mr Mugabe.
Mr Brown did not explicitly call for Mr Mugabe to step down, but said world leaders should stand together to defend human rights and democracy in Zimbabwe.
In a statement, Mr Brown said that over the coming days the first priority would be delivering aid to Zimbabwe, such as rehydration and testing packs.He said: "This is now an international rather than a national emergency. International because disease crosses borders.
"International because the systems of government in Zimbabwe are now broken. There is no state capable or willing of protecting its people.
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"International because - not least in the week of the 60th anniversary of the universal declaration of human rights - we must stand together to defend human rights and democracy, to say firmly to Mugabe that enough is enough."
Mr Brown said he had "been in close contact with African leaders to press for stronger action to give the Zimbabwean people the government they deserve".
He added: "The people of Zimbabwe voted for a better future. It is our duty to support that aspiration."
He also said he hoped the United Nations Security Council would meet "urgently" to consider the country's plight.
Mr Brown has joined a growing list of international leaders in condemning Mr Mugabe.
US Secretary of State, Condoleezza Rice, said it was "well past time" for him to leave office.
And the Nobel Peace Prize laureate, Archbishop Desmond Tutu, said African nations should come together to use military force if Mr Mugabe refused to go.
Archbishop Tutu said Mr Mugabe had committed "gross violations" against Zimbabwe's people and ruined "a wonderful country".
UK International Development Secretary Douglas Alexander told BBC Two's Newsnight that he welcomed the archbishop's comments, but dealing with the humanitarian crisis was of paramount importance.
He said there was not currently a consensus on the UN Security Council over Zimbabwe, even for further targeted sanctions, but said "the new noises that are emerging from Africa this week" were welcomed by the UK.
Mr Mugabe and opposition leader Morgan Tsvangirai agreed to share power in September to tackle the country's economic meltdown but they have been unable to agree on the allocation of cabinet posts.