The ICC yesterday said Ruto could only be excused under "exceptional circumstances," reversing a July ruling that excused him from most of his trial before the Hague court.
Judge Sang-Hyun Song said July's ruling had "provided Ruto with a blanket excusal even before the trial commenced, effectively making his absence a general rule and his presence an exception".
Ruto and Kenya's President Uhuru Kenyatta are both charged with crimes against humanity in connection with the 2007-8 post-election violence in Kenya that left over 1,000 people dead and several hundred thousand displaced.
But the president's lawyers have asked for the hearing to be put back to next February, citing the "national and international crisis" triggered by a deadly attack on a Nairobi shopping centre.
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They argued that last month's militant attack on Nairobi's Westgate shopping mall by Al-Qaeda-linked militants in which at least 67 people died meant he was urgently needed at home.
The attack was carried out by neighbouring Somalia's Shebab, which has threatened to launch further attacks.
Kenyatta, who was elected president in March, has long argued that his trial would hamper his running of the country.
The Kenyan president is currently only required to be in court for the trial opening, the verdict and when victims are giving testimony against him.
However, on Friday, chief ICC prosecutor Fatou Bensouda said she would appeal an excusal granted to Kenyatta so he could fulfil his "demanding" political duties in the wake of Nairobi's deadly Westgate shopping mall attack.
But the ICC has been caught up in accusations that it is targeting African leaders, with the African Union calling for immunity for heads of state and asking the UN Security Council to suspend ICC proceedings against Kenya's leaders for a year.
Ruto, 46, and Kenyan radio boss Joshua arap Sang, 38, stand accused of stoking the worst violence in the east African country since independence in 1963.
Victims of the post-election violence had opposed the ruling excusing Ruto from most of his trial, which required his absence to be "seen to be directed to his duties of state".