Mr Justice Deeny said Stone had been "a wholly unreliable and unconvincing witness whose testimony… is wholly undeserving of belief".
He concluded Stone had not merely prepared the explosive devices and gone to Stormont estate, but had "stepped over a line by entering the building and igniting the bag which was a necessary part of his plan to kill the Sinn Fein leaders".
He had therefore gone beyond acts that were merely preparatory.
The judge also noted Stone had repeatedly told police when arrested that his intention was to kill Gerry Adams and Martin McGuinness, including that he would have slit their throats if he had got close enough.
The defendant had also written letters to two journalists, which he posted on his way to Stormont, saying this was his intention.
In 2000, Stone was released early from a life sentence under the terms of the Good Friday Agreement.
He had been jailed for a 1988 gun and grenade attack on the republican funeral at Milltown Cemetery of three IRA members shot dead by the SAS in Gibraltar.
Three men were killed in that attack and Stone also admitted three other murders.
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