Tony Blair has "failed to deliver" on his promises to help deliver peace to the Middle East, Christian Aid said.
The leading charity said Mr Blair's policy in the region, particularly his "refusal" to engage with the Hamas-led National Unity government, had helped to "exacerbate" the current situation.
The charity has called on Gordon Brown not to make the same mistakes and urged him to lead a "renewed drive for peace".
Janet Symes, Christian Aid's regional manager for the Middle East, said: "Mr Blair, who steps down as Prime Minister later this month, promised in December last year that peace between the Israelis and Palestinians would be his legacy.
"But it is quite clear from the disastrous situation in Iraq and the implosion of the Gaza Strip, that the Prime Minister has failed to deliver in these areas."
The charity said the public was "paying heavily" for Mr Blair's mistakes by funding increasing aid when a political solution was needed.
Currently, up to 80 per cent of families in Gaza do not have an income and instead rely on UN aid for food and other necessities.
She added: "Mr Blair has failed to argue that a year of sanctions against the Palestinian Authority should end immediately. And he has refused to engage with the National Unity government.
"Christian Aid argues that this isolation has contributed to the collapse of the Gazan economy which has helped create the climate of desperation and increased the severity of the internal conflict."
Christian Aid has written to Foreign Secretary Margaret Beckett to urge her to use a meeting with other EU ministers this week to help improve the situation.
