The officers were returning to the headquarters of the Nottinghamshire force at Arnold, Nottingham, on Wednesday after a session at a police shooting range at Epperstone Manor.
A black canvas holdall containing the five unloaded Smith and Wesson guns slid out of the van during the journey.
Nottinghamshire Assistant Chief Constable Michael Todd denied yesterday that the five officers had been suspended but confirmed their authorisation to use firearms in police work had been withdrawn until the outcome of an inquiry.
The officers are said to be distraught and are likely to face disciplinary hearings.
Mr Todd said: "These people are all highly trained and all of us are taking this very seriously. We just want those guns back.
He said the constables were from "the operational support unit of the force which is a firearms training group".
"It appears they put them on the floor of the vehicle and because of the hot weather hadn't shut the side door. They must have slid along the floor of the van and out of the door.
"The officers discovered they were missing before they got back when they were about three-quarters of the way along the five-mile journey and turned round immediately and went over the route."
However, the guns and bag had disappeared and it was thought that a couple passing by in a car had picked them up.
Mr Todd said: "One of the officers remembers a red MG Metro car flashing its lights at the police vehicle and a farm worker in the area has told us that he saw a couple leave the same car and pick up a black bag.
"If that is the case, you have to ask why haven't they taken them into the nearest police station?" He added that the guns and bag were not marked with police identification.Reuse content