He took over command of the army's 59th division on or around 30 April 2009, for advancing Sri Lankan troops from Mullaitivu North to Mullivaikkal to capture Vadduvakkal causeway.

[6] As commander of the 55th division of the Sri Lankan army in the final months of the Sri Lankan war, de Silva has criminal liability (command responsibility) for genocidal acts, war crimes and crimes against humanity committed by his troops against Tamil civilians and LTTE cadres in these final months.

These mass atrocity crimes mainly occurred in "No-Fire Zones", which were areas unilaterally announced by the Sri Lankan government for Tamil civilians to congregate in order avoid shelling and artillery fire,[1] but in reality were areas of premeditated massacre of tens of thousands of Tamil civilians by the Sri Lankan government.

[7] International crimes for which Prasanna de Silva is responsible include: After the war, de Silva served as Defence Attaché to the Sri Lankan Embassy in London, UK (September 2010 – April 2012), possessing diplomatic immunity that prevented him from being prosecuted for his role in the Sri Lankan army's mass atrocity crimes.

Following submission, the FCO refused to declare de Silva as "persona non grata", allowing him to avoid prosecution and leave for Sri Lanka in April 2012.