Gordon Brown is holding Downing Street talks with Palestinian president Mahmoud Abbas amid efforts to restart the Middle East peace process.
The two leaders will consider how to move forward in the wake of Israel's controversial offensive on the Gaza Strip.
Speaking at the European Parliament in Strasbourg on Wednesday, Mr Abbas called for a war crimes probe into the military action.
"People who committed those crimes have to be held responsible so that these crimes cannot be repeated," he said after a speech which garnered a standing ovation. "We are stretching out our hand for peace with Israel. But what was done is regretfully crimes of war."
The head of the Palestinian Authority also denounced Hamas's rocket attacks on Israel, which Tel Aviv insisted motivated their assault.
However, Mr Abbas said the "doors are still open" for a unity government with Hamas.
Meanwhile, Hamas denied claims by United Nations officials that it had seized aid supplies meant for civilians in Gaza.
Members of the group were said to have raided 3,500 blankets and 400 food parcels from a UN warehouse in the territory.
