Monmouth MP David Davies has been asked to resign his role as a special constable after nine years of service for the British Transport Police.
New guidance issued in the College of Policing’s code of ethics states a special constable “should not take an active role in politics.”
The policy has forced Mr Davies to leave his part-time role alongside fellow special constable, Kettering MP Philip Hollobone.
He said it was a “pity the rules had changed” and was “saddened” by the decision.
Mr Davies joined the British Transport Police in 2006 when he was a member of the Home Affairs Committee. He worked a shift as a volunteer special constable about once a fortnight in London.
“I’m disappointed to stand down because I have had a lot of policing experience which I have brought into the House of Commons Chamber,” said Mr Davies.
“Obviously, I respect the decision and don’t intend to argue against it but I think it’s useful for MPs to have their feet in the real world in some way.
“It’s a great shame I am no longer allowed to do this.”
During his service, Mr Davies patrolled the streets of London when rioting escalated in 2011 and helped out with security for the 2012 Olympics and Notting Hill Carnival. He also worked with a plain clothes anti-pickpocket squad in Brixton.
In 2008, Mr Davies received a letter of commendation from BTP for his arrest of a man for attempted murder.
Detective Chief Superintendent Miles Flood wrote: “Through your courage of conviction, you have contributed heavily in building a strong prosecution against the suspect and helping ensure justice for the victim who was stabbed 11 times."