Oxfam, Christian Aid and Cafod have written to the Prime Minister to seek assurances that the British Government is still committed to the complete withdrawal of Israel from the occupied Palestinian territories.
They were responding to Israeli Prime Minister Ariel Sharon's promise last week to withdraw from the Gaza Strip but retain some settlements on the West Bank.
US President George Bush has backed Sharon's plan and Tony Blair has described it as an "important step" towards peace in the Middle East.
But the aid agencies want an assurance that the UK Government still sees Israeli settlements in Palestine as illegal and obstacles to peace.
"We are gravely concerned the Government will alter its policies in line with the Bush announcement," states the letter. "Bush's support for the policy of unilateral action is in breach of international law and could usher in a new era of instability."