KUALA LUMPUR (Reuters) - Electoral watchdog groups in Malaysia said on Thursday the voter list for next week’s general election had major flaws, including the existence of a 121-year-old voter, raising the specter of possible fraud.

FILE PHOTO: Barisan Nasional and Malaysian Islamic Party (PAS) flags hang at a public housing estate in Kuala Lumpur, Malaysia April 20, 2018. Picture taken April 20, 2018. REUTERS/Stringer

About 15 million Malaysians are registered to vote in Wednesday’s election pitting Prime Minister Najib Razak’s Barisan Nasional coalition, which has ruled for six decades, against a resurgent opposition led by former leader Mahathir Mohamad.

A joint study of the voters’ rolls by electoral reform groups Bersih and Engage found more than 500,000 cases of voters registered with the same address, while more than two million were found to have no address.

The groups highlighted 10 major irregularities they said affected hundreds of thousands of voters nationwide.

“A defective electoral roll will bring into question the legitimacy of the whole election,” they said in a statement.

“Despite the huge number of dubious voters discovered, we believe these preliminary findings are just the tip of the iceberg.”

The study found some cases in which dead voters were re-registered, and one voter whose birth year was listed as 1897.

In one example, the study listed numerous cases of multiple people registered at the same address in the parliamentary constituency of Bagan Datuk, held by Deputy Prime Minister Ahmad Zahid Hamidi.

“Our conclusion is that there are symptoms of a deliberate plan ... action to massively move voters to impact elections in marginal constituencies,” Bersih official Chan Tsu Chong told a news conference.

The groups did not say who they believed was behind the plan.

The Malaysian Election Commission did not respond to a request for comment.

Barisan Nasional said in an email response that Bersih’s findings should not be taken as objective facts as it was a “functioning arm of Malaysia’s opposition”.

“As such, Bersih has every reason to undermine confidence in the vote so that afterwards it can complain that the election wasn’t fair. This is a smear on Malaysia, and the integrity of Malaysia’s independent Electoral Commission,” a spokesperson said.

The opposition and other critics have said that recently redrawn electoral boundaries favor the ruling coalition, which has been accused of gerrymandering.

The government and the Election Commission have rejected that assertion, saying the changes were made independently and without political interference.

Campaigning kicked off last Saturday for an election that is widely expected to be the toughest yet for Barisan Nasional.

Najib is grappling with a multi-billion-dollar scandal at state fund 1Malaysia Development Berhad (1MDB) and popular anger over rising living costs, which have given momentum to the challenge from his 92-year-old mentor-turned-foe, Mahathir. Najib denies wrongdoing.

The coalition is expected to retain power, but a diminished majority in the 222-seat parliament could leave Najib open to an internal leadership challenge.

Bersih and Engage said in their statement that the effect of the “cheating” could only be overcome by an overwhelming voter turnout.