Karat was addressing a seminar to mark the 10th four-day conference of the West Bengal unit of CITU beginning here on November 8.
With none but NCP on the Congress' side, the ruling party at the Centre will find it really difficult to get the bill passed, he said.
On Left's opposition, Karat said, "We will continue to oppose and agitate if FDI investment takes place in any retail unit anywhere".
"There are an estimated four crore people depending on the retail sector in the country, the second highest employment generating sector after agriculture. We cannot take it lying down if their livelihood is at stake," the CPI(M) ideologue said.
Karat said the second UPA government was in effect abetting loot of the common man in the name of liberalisation.
Former state Industry Minister Nirupam Sen said foreign investment was welcome for upgrading technology of a company but not in sectors like FDI.