The government has been accused of "quietly plotting" to end the European Objective One funding for Cornwall. St Ives Liberal Democrat MP Andrew George claims the government will allow all European aid to go to new EU states and other English regions after 2006.
He said such a move would cost Cornwall about �500m.
Falmouth and Camborne Labour MP Candy Atherton said Chancellor Gordon Brown guaranteed Cornwall would not lose out and the Lib-Dems were scare-mongering.
Mr George will makes his claims during a Commons adjournment debate on Wednesday night.
Objective One aid is available to EU areas where prosperity, measured in Gross Domestic Product (GDP) per head of population, is 75% or less of the European average.
The current European-funded programme is due to end in 2006. By then, an estimated pot of �300m is expected to have been spent.