Somawansa Amarasinghe, leader of the Marxist JVP says the deal is a violation of the sovereignty of the administration.
President Chandrika Kumaratunga's government has a five-seat majority in the parliament and depends on the 39 votes of the JVP.
JVP says it is not against Tamil people receiving tsunami aid, but says there is no need to do a deal with the Tigers to get relief through.
Buddhist monks are also opposed to the deal and their sit-down protest entered its third day Friday.
The president's critics say the plan to jointly distribute international tsunami relief aid with the Tamil Tigers will help legitimise the rebels' separatist campaign.
International donors have called for a joint mechanism to distribute billions of dollars in aid equitably in rebel-held and government areas.
Some 31,000 people were killed in the December 26 tsunamis and a million people were initially left homeless.
Much of the destruction was in the northeast, parts of which are dominated by the guerrillas.
- Australian Broadcasting Corporation
- © ABC