Minister counsellor Priyanka Fernando, who is also a serving officer in the Sri Lankan army, was relieved of his duties pending a disciplinary inquiry over Sunday’s incident.
Videos of Fernando making throat-slitting gestures were shared widely on social media, embarrassing Sri Lankan authorities who have promised reconciliation after decades of ethnic civil war.
“Authorities in Sri Lanka have taken serious note of videos being circulated... involving the minister counsellor behaving in an offensive manner,” the ministry said.
Fernando, who is a brigadier, was involved in fighting during the bloody finale to the island’s Tamil separatist war in 2009, when Sri Lankan troops were accused of massive human rights abuses.
The Tamil protesters in London were demonstrating against the continued occupation of private Tamil land in the island’s war-scarred north, nine years after the end of fighting.
An officer “wearing the much-hated military uniform ran a finger across his neck... a gesture that he would slit our necks,” one of the participants wrote on the protest organisers’ website.
The rebels – known as the Tamil Tigers – were crushed in a no-holds-barred offensive that sparked widespread allegations that as many as 40,000 Tamil civilians were killed by security forces.
The current government, which came to power in January 2015, has promised to investigate the alleged atrocities and release private land occupied by the military in former war zones, but progress has been slow.