The Constitutional Court ruled unanimously on Tuesday that the Senate election did not breach the charter, clearing the way for the poll to proceed to its final step next week.
All nine judges on the panel agreed that four sections of the organic law on the composition of the Senate election did not violate the constitution, as claimed by six petitioners, five of whom are Senate candidates.
Following the court's ruling, the Election Commission (EC) confirmed that the national-level Senate election will take place on June 26, as originally scheduled.
The final phase of the election will be held at IMPACT Forum, a convention centre in Pak Kret district in Nonthaburi.
Winners from the district and provincial levels must first select candidates within their own groups and then cross-select those from outside their groups.
In the last stage, 3,000 winners representing 20 groups from the previous rounds will vie for 200 seats in the Upper House.
On Monday, caretaker senator Somchai Swangkarn urged the EC to expedite an investigation into alleged irregularities in the weekend provincial-level Senate election after some candidates received an unusually high number of votes from their peers in Buri Ram.