The Election Commission, whose members are picked by the government, has said it intends to reorganise boundaries in 112 parliamentary seats -- about half the national total -- as well as 445 of the 576 state seats.
Opposition lawmaker Tony Pua from the Democratic Action Party said the new boundaries are designed to ensure a win for Najib who is embroiled in a massive corruption scandal involving state investment fund 1MDB.
He said the rearrangement aims to bundle opposition- inclined voters into "super-constituencies" with more than 100,000 voters, while breaking up ruling party supporters into smaller, multiple constituencies.
The move would favour the dominant United Malays National Organisation which leads the 13-party ruling Barisan Nasional coalition, by allowing it to win maximum seats with the least possible votes, opposition politicians said.
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Bigger political constituencies could also hobble the opposition which has limited financial and manpower resources to garner votes.
The 63-year-old premier is clinging to power despite increasingly damaging allegations that he took part in the looting of billions in state funds. Najib and 1MDB have repeatedly denied wrongdoing in the scandal.
UMNO has been in power in Malaysia since independence in 1957.
"This systemic rigging been going on for the past few decades," said James Chin, the director of the Asia Institute at the University of Tasmania.
"There is no doubt this is blatant gerrymandering. Najib is set to win again soon even before the first vote is cast."