The jury of seven men and five women took just under four hours to return a majority verdict against Tariq Jahan.
Jahan was cleared of the more serious charge of causing grievous bodily harm with intent.
His Honour Judge William Davis QC will sentence him this afternoon.
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It comes as Labour leader Jeremy Corbyn reiterated his call for MPs to work together to prevent a no-deal withdrawal from the EU.
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Jack Letts, also known as 'Jihadi Jack', has reportedly been stripped of his UK citizenship.