The opposition Democratic Action Party (DAP) says the election commission's refusal to allow nearly 700,000 newly registered voters to take part is unconstitutional.

The party says this unfairly excludes nearly 7% of the electorate - mostly young people thought to be sympathetic to the opposition parties.

The BBC's Frances Harrison: "The challenge could theoretically stop the elections from taking place".

The secretary-general of DAP, Lim Kit Siang said the electoral commission was complying with the wishes of Malaysian Prime Minister Mahathir Mohammed in disenfranchising 680,000 voters.

"This has brought shame and infamy to Malaysia in dragging out the voters' registration exercise," he said.

'Mahathir for PM' badges

The election commission has said it takes nine months to update the electoral roll to include the new voters.

The opposition parties describe it as scandalous that the process should take so long in an era of information technology and electronic government.

Commenting on the possibility that the legal challenge might not be heard until after the poll, a lawyer representing DAP said, "If we succeed in the end, it means that the elections will still be void."

'Deliberate' exclusion of young voters

Opposition leaders have argued that the snap election was called my Mahathir Mohammed deliberately to exclude this group of new voters.

Anwar: rallying call inspires the young

The commissions decision, they say, is part of the prime minister's tactics.

Correspondents say the dramatic events of the past 15 months since the sacking and trial of deputy Prime Minister Anwar Ibrahim have politicised previously apathetic voters in Malaysia.

Three times the average number of new voters registered in April and May this year.

Opposition parties believe many of these are young Malaysians inspired by the Mr Anwar's rallying call of political reform.