Thailand's Constitutional Court has ruled the 2 February general election invalid, officials say.
The snap poll was called by Prime Minister Yingluck Shinawatra amid major anti-government protests in Bangkok.
The ruling party was expected to win, but the opposition boycotted it and protesters disrupted voting, meaning the election has not been completed.
The vote was unconstitutional because it did not take place on the same day across the country, the court said.
Polls were not held in a number of constituencies because protesters had blocked candidate registration.
The court, which ruled to void the election by six votes to three, was responding to a motion by a law lecturer who had challenged the election on a number of points.
It is not clear when a new election will be held.
But protest leader Suthep Thaugsuban said another election would face similar problems.
"If a new election date is declared, then we'll take care of every province and the election won't be successful again," he said late on Thursday.
A spokesman for the ruling Pheu Thai party, Prompong Nopparit, said the party would play by the rules "no matter how much we are bullied".
"The reason this election is nullified is because the polls were blocked by the protesters, weren't they? We've played by the rules all along, but what about the other side?" the Associated Press quoted him as saying.
The anti-government protests began in November 2013. At the height of the demonstrations, protesters shut down key road junctions in Bangkok and blockaded government ministries.
Numbers have fallen in recent weeks, however, and the protesters are now mainly occupying a city-centre park.
At least 23 people have died and hundreds have been injured in the course of the demonstrations.
The protesters, who are mainly urban and middle class, want Ms Yingluck's government replaced by an unelected "people's council".
They allege her brother, ousted leader Thaksin Shinawatra, controls her administration and say Shinawatra family money has corrupted Thai politics.
Ms Yingluck and Pheu Thai remain very popular in rural areas, however, leaving Thailand deeply polarised.
These are the worst protests to hit Thailand since 2010 when "red-shirts" - those who supported Mr Thaksin and opposed the then opposition-led government - shut down parts of Bangkok for several weeks.
About 90 people were killed over the course of those protests, which were ended by a military crackdown.
The South East Asian nation has been embroiled in a cycle of political unrest since the military removed Mr Thaksin from office in a 2006 coup.