Malaysian authorities have rejected a plea to drop the murder case against Doan Thi Huong, the Vietnamese woman accused of killing the half-brother of North Korea's leader.
Vietnam had been pushing for her release, and the decision comes days after her Indonesian co-defendant, Siti Aisyah, was unexpectedly freed.
Both women insist they are innocent.
Kim Jong-nam was assassinated with liquid VX nerve agent at Kuala Lumpur airport in February 2017.
Ms Huong says she was tricked into taking part and believed she was part of a TV prank.
The 30-year-old, who could face the death penalty if convicted, sobbed in court when it was announced that her trial would go ahead.
She told reporters: "Only God knows that we did not commit the murder. I want my family to pray for me," AFP news agency reports.
Her lawyer said she had been unable to sleep since the unexpected release of Siti Aisyah, who flew to Jakarta the day she walked free from court.
Ms Huong, who arrived at court in a bulletproof vest and red headscarf, has already been on trial for a year and a half.
The presiding judge said the defendant was not "physically and mentally" well enough to continue with the proceedings.
Vietnam's government said earlier that Foreign Minister Pham Binh Minh had asked Malaysia to "ensure a fair trial for Huong and set her free". Vietnam's justice minister had also written to Malaysia's attorney-general asking for her to be freed.
Kim Jong-nam, the estranged half-brother of North Korea's leader Kim Jong-un, had been waiting to board a flight from Kuala Lumpur to Macau on 13 February 2017 when two women approached him in the departure area.
CCTV footage showed one of them placing her hands over his face, then both women leaving the scene.
Mr Kim died on the way to hospital from what was later found to be exposure to the nerve agent VX, one of the most toxic of all known chemical agents.
North Korea has fiercely denied any involvement in the killing, but four men - believed to be North Koreans who fled Malaysia on the day of the murder - have also been charged in the case.
They remain at large despite an Interpol "red notice", equivalent to an international arrest warrant.