The Asian G4S worker flipped as he patrolled an archery contest at Lord’s cricket ground with the Afghan veteran.
A source said: “This security guard was ranting and raving at the soldier.
“He was trying to defuse the situation but the guard was going crazy. He spat at the soldier and screamed at him. He was calling him a baby killer.
“He left the soldier in no doubt he meant he’d been responsible for children’s deaths while serving in Afghanistan.
“Despite the provocation, the soldier remained ice cool and didn’t lose his calm.
“Some of their respective colleagues arrived and the guard was removed.”
Bosses at security company G4S — which had to be bailed out by troops after failing to supply enough guards for London 2012 — yesterday said an urgent investigation was under way.