A 60–year old diabetic patient has an uneventful phacoemulsification with IOL implantation. He presented with pain, diminished vision, redness, watering and a greyish yellow pupillary reflex on the third postoperative day. Examination revealed circumcorneal congestion, aqueous cells 4+, hypopyon in the anterior chamber with posterior synechiae and retrolental flare. The most likely diagnosis is –