The country’s Constitutional Court voted unanimously on Thursday to hear the cases against prime minister Thaksin Shinawatra’s ruling Thai Rak Thai party and the opposition Democrat Party.
Three other minor parties involved in political controversy during April's, now annulled, general election will also be investigated by the court.
The election was boycotted by the Democrats and other opposition parties after weeks of anti-Thaksin street protests.
They subsequently accused Thai Rak Thai of trying to prevent its candidates from falling foul of minimum turn-out laws by paying smaller parties to field candidates in constituencies with uncontested seats.
The Election Commission says the Democrats paid members of the smaller parties to make the allegations against Thai Raik Thai, and also accuses Thailand's oldest party of encouraging voters to spoil their ballots.
The Commission itself is accused of favouring Thaksin's party, and has been taken to court accused of mishandling the elections.
The courts have come under pressure to navigate a way out of the political deadlock after annulling the April 2 vote.
Protests have been common
in Thailand recently
However, the judges have made little headway amid a flurry of lawsuits from all sides. A new election has been tentatively scheduled for October 15, but the opposition and many political analysts think it will have to be pushed back further.
The latest decision by the Constitutional Court's 15 judges came amid heightened tension as Thaksin said the National Intelligence Agency had unearthed a plot to kill him.
"There was intelligence but it was not frightening. It's nothing unusual," he told reporters.
Thaksin said his security team has been reinforced since the threat surfaced last week and the plot was being investigated.
The accused political parties meanwhile have 15 days to send their defence to court.