Cameron said the violence witnessed in Kiev this week, in which nearly 100 protesters have died, had "no place in 21st century Europe".
Ukrainian President Viktor Yanukovych has agreed to form a new unity government and hold early elections.
"I welcome today's agreement which offers a real chance to end the bloodshed and to stop the downward spiral into the nightmare that is facing Ukraine and her people," Cameron said in a statement.
Cameron said he hoped the deal "will restore calm to the streets of Kiev and across the country".
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"Such scenes of carnage are truly shocking and have no place in 21st century Europe."
Cameron has spoken to Russian President Vladimir Putin, German Chancellor Angela Merkel and Polish Prime Minister Donald Tusk in the last 24 hours.
"We all support this deal and want to see it work," Cameron said.