Mark Francois was spotted wearing the equipment on Friday night in the market town of Rayleigh.
Alongside a picture of the MP, one Twitter user wrote: “Walking around town and in and out of pubs, dressed as a police officer and wearing a stab vest.”
She added: “He was met with some ridicule in the pubs as you can probably imagine!”
The photograph was taken in The Roebuck, a Wetherspoons pub on Rayleigh High Street.
Some questioned his choice of attire while one commenter asked: “Genuine question, can you be arrested for impersonating half a police officer?”
However, Essex Police confirmed that the 54-year-old MP for Rayleigh and Wickford had been given the wrong protective vest to wear during the outing.
“He joined our officers in Rayleigh to see the work they are doing in #Essex and we gave him the wrong jacket,” the force wrote on Twitter.
Mr Francois, the deputy chair of the hardline European Research Group, is an ardent Brexiteer.
“I was in the army. I wasn’t trained to lose,” he once said, during an interview about Brexit.
Support free-thinking journalism and attend Independent events
It later emerged that he had served as an infantry officer in the Territorial Army during the Cold War.