Sources at the Department of Foreign Affairs said Guingona, who was invited to the event, was asked by protocol officers to leave the luncheon a few minutes after his arrival because his name was not on the guest list.
One of the DFA sources said Guingona was supposed to be in that function because he had an invitation.
"He would not have attended the function if he was not invited by its organizers as dictated by strict diplomatic protocol," another DFA source said.
Humiliated for being mistaken for a gatecrasher and for being asked to leave the function, Guingona immediately left the room without showing his invitation to the luncheon.
"He just left the room and did not say anything. It was amor propio," the source added.
Guingona immediately sent a cable to the Home Office the same day, instructing the DFA to file a diplomatic protest against Mexico for the incident.
"We do not know if it was the fault of the organizer, but we need an explanation about the incident," another source said.
The government is reportedly disappointed over the unkind treatment in Mexico of Guingona, the second highest official of the country, who has gained respect for his nationalism and statesmanship.
The DFA will summon the Mexican ambassador tomorrow to discuss the embarrassing incident. The note verbale is expected to be handed to the Mexican envoy at the DFA.
Guingona left last Sunday on official trip to Mexico to attend the United Nations International Conference on Financing for Development. He is expected to return to Manila tomorrow.