Details only emerged this week due to “operational reasons” but the men were arrested in May during the “initial stages of enquiries”, according to the City of London Police.
The men were arrested on suspicion of conspiracy to defraud, and two other men were interviewed under caution. Search warrants were also executed throughout the UK.
The force said it was asked to support the Food Standards Agency (FSA) with its inquiry into how products became adulterated with horse meat.
Oliver Shaw, detective chief superintendent, from the City of London Police, said: “This is an extremely complex investigation covering a number of jurisdictions and a variety of businesses.
“We are working closely with police forces, other law enforcement agencies and regulators to determine whether horse meat being used in a range of meat products was deliberate and coordinated criminal activity.”