A Myanmar court found Aung San Suu Kyi guilty of electoral fraud, sentencing her to three years' imprisonment on Friday.
The ruling adds more jail time to the 17 years she is already serving for other offenses, including corruption and incitement.
It also throws the future of her National League for Democracy (NLD) party into jeopardy as the military government had threatened to dissolve it before the promised 2023 elections.
Co-defendant Win Myint, the deposed president, and Min Thu, the former minister of the president's office, were given the same sentence.
Many other NLD leaders are also imprisoned or have escaped abroad.
Suu Kyi's party won a resounding victory in the November 2020 elections in Myanmar, trouncing a party created by the military.
Accusing the NLD leader of orchestrating widespread voter fraud the military staged a coup in February 2021 to stop Suu Kyi from forming a new government.
International observers assert that the elections were largely free and fair, refuting the military claims of more than 11 million instances of voter fraud.
Last month, junta chief Min Aung Hlaing said that fresh polls would only be held once the country is "peaceful and stable."
Since the military coup, more than 2,000 people have been killed and another 17,000 arrested in a crackdown on opposition and pro-democracy activists, according to local monitoring groups Assistance Association for Political Prisoners.
Rights groups have condemned the arbitrary arrests and detentions of activists and peaceful protesters. They accuse the junta of denying people the rights to freedom of expression and peaceful assembly.
Suu Kyi had initially been jailed after being found guilty of violating the country's coronavirus pandemic rules as well as illegally importing walkie-talkies.
Then in the first of a series of trials over alleged corruption, Suu Kyi was jailed for a further five years in April. An August conviction added six more years to the sentence.
The trials have been held behind closed doors in the capital Naypyitaw.
The combined maximum sentences for all the charges against the 77-year-old former leader is more than 190 years.
She denies all the allegations against her, while her supporters call them politically motivated.