In August Nuon Chea, 88, known as "Brother Number Two", and ex-head of state Khieu Samphan, 83, were given life sentences for crimes against humanity after their first trial at Cambodia's UN-backed court.
That ruling saw them become the first top figures to be jailed from a regime responsible for the deaths of up to two million Cambodians from 1975-1979.
Both men lodged appeals against the convictions last month but their lawyers today said they needed time to file the full appeal documents.
It is the first time defence lawyers have staged a boycott of the special court, set up in 2006 to try senior regime figures, in a move which could further prolong the trial.
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The complex case against Nuon Chea and Khieu Samphan was split into a series of smaller trials in 2011 in a bid to get a faster verdict for reasons including their advanced age and the large number of accusations.
The August convictions followed a two-year trial focused on the forced evacuation of around two million Cambodians from Phnom Penh into rural labour camps and murders at one execution site.
Prosecutors opened their arguments yesterday in a second, broader, trial in which the pair are charged with genocide as well as further crimes of humanity and war crimes.