There is fresh confusion over when Thailand will hold new elections, and over who can decide the date.
Thailand's senior judges don't want the Election Commission involved
The country's three top courts have rejected a proposal made earlier this week by the Election Commission for the new poll to be held in October.
They also repeated their call for the commissioners to resign over their alleged mishandling of the last poll - annulled by the courts a week ago.
The 2 April poll results were scrapped because the opposition boycotted it.
Meanwhile Prime Minister Thaksin Shinawatra has begun resuming some official duties.
Despite announcing a few weeks ago that he would step aside, bruised by a large number of protest "no votes" in the April elections, he was spotted arriving at Government House late on Monday for a meeting about the country's upcoming royal anniversary.
"Prime Minister Thaksin is now involved with many issues, such as arranging state ceremonies to mark the king's 60th year on the throne," said Deputy Prime Minister Chidchai Vanasathidya.
Mr Thaksin has yet to announce whether he will stand as prime minister again in the next elections.
The Supreme, Constitutional and Supreme Administrative Courts met on Tuesday to discuss the Election Commission's proposed October date.
"The courts consider the meeting initiated by the EC to set a date for elections was illegal," said Supreme Court Justice Charan Pakdithanakul, explaining that not all the commissioners were present when the decision was made.
Correspondents say the judgment adds yet another complication to the country's political crisis.
Thailand currently has no functioning parliament and only a caretaker government - a situation which will remain until fresh elections are held.
The Election Commission has received much of the blame for the flawed poll in April, and there is increasing pressure on the commissioners to quit.
"If the EC does not resign, then the courts cannot guarantee a free and fair election," Mr Charan said on Tuesday.
One of the four commissioners is already reported to have left his post.