Hamas warned Tony Blair on Saturday his credibility as the new international Middle East peace envoy will be damaged if he ignores the organization. The former British prime minister was expected in Israel and the West Bank early next week in his maiden visit since his appointment as envoy of the diplomatic Quartet - the United States, European Union, United Nations and Russia. "We warn the new envoy that any attempt to marginalize the Hamas movement will cost him his credibility," said Hamas hard-liner Mahmoud Zahar, a former Palestinian Authority foreign minister. The Quartet refuses to deal with Hamas because it refuses to recognize Israel's right to exist, renounce violence or accept agreements by previous Palestinian governments with Israel. That policy won't change to accommodate Blair, Zahar said. "We are not ready to sit with anyone calling on us to abandon our national constants and to foreswear the aspirations of the Palestinian people," he said.