Saddam Hussein personally signed documents ordering the killing of 148 Shia villagers in Dujail, handwriting experts have concluded.
Saddam Hussein listened quietly as the findings were announced
He and seven co-accused face charges for their alleged role in the killings after an assassination attempt in 1982.
Prosecutors have presented thousands of documents to the court to try to prove a paper trail exists linking the former Iraqi leader directly to the killings.
Defence lawyers have insisted the signatures are a forgery.
They have also contested the impartiality of the handwriting experts, who they say are linked to Iraq's current interior ministry.
The BBC's James Reynolds in Baghdad said the experts' decision was a very significant moment for the prosecution.
Chief judge Raouf Abdel Rahman had ordered a two-day delay after the previous session earlier this week to allow them more time to evaluate the authenticity of the signatures.
Among the documents was one, from 16 June 1984, apparently approving the Dujail executions and another from 10 October 1982 - three months after the attempt on Saddam Hussein's life - authorising rewards for intelligence agents involved.
The judge opened Wednesday's session by announcing the experts' verdict.
Saddam Hussein's half brother headed the intelligence service
"The experts verified these documents and the signatures of Saddam Hussein were found to be authentic," he told the court.
Documents apparently bearing the signature of former intelligence chief Barzan Ibrahim al-Tikriti, Saddam Hussein's half-brother, were also ruled to be genuine.
After a three-hour session, the trial was adjourned until 24 April, so the experts could spend more time examining the signatures of co-defendant Mizher Abdullah Rawed.
While Saddam Hussein sat quietly throughout the Wednesday's session, Barzan al-Tikriti loudly contested the documents' authenticity.
"[The] prosecutor is clearly biased; he is using any means to make the accused guilty," he declared.
The documents, which have not been released publicly, are said to include Saddam Hussein's signature on a "death warrant" ordering the execution of the 148 Shia villagers.
They were tried by Iraq's Revolutionary Court in 1982 and later executed, allegedly with direct authorisation from the country's president.
Prosecutors have argued that the brutal response was unjustified even by an assassination attempt.
At earlier hearings, Saddam Hussein acknowledged signing execution orders, saying it was his duty as president of Iraq. But he later appeared to dispute their authenticity.
If convicted, Saddam Hussein is expected to face the death penalty.
Earlier this month the court announced he would face new charges of genocide and crimes against humanity.
Those charges relate to the Anfal military campaign against Kurds in northern Iraq in the late 1980s, in which as many as 180,000 people may have died.
The case will be tried separately.