Hertsmere MP James Clappison has announced he will not be standing for re-election in 2015.
The Conservative, who has served as MP for the safe seat for 23 years, has written to David Cameron today explaining his decision.
In the letter he said that he will always be loyal to the party but that it is "time to move on."
The backbencher said: "It has been a great honour to represent the great people of Hertsmere since I entered the House in 1992.
"I served as a Minister under Sir John Major and remained on the Conservative frontbench for seven years until I joined the Home Affairs Select Committee in 2002. I was delighted to serve as a DWP spokesman as part of your team before the last General Election. Overall my time in the House has left me with many warm and fond memories, for which I will always be grateful.
"By May 2015, I will however have served as a Member of Parliament for a full 23 years: a generation of time. It is difficult letting go but I do feel now is the time to move on. There is a world elsewhere.
"I will always remain personally loyal to you and to our historic mission as a Conservative Party: to secure Britain's economic future, and to give every young person growing up today the chance of making something of their lives. I look forward to working with my successor to help ensure the election of a new Conservative MP in Hertsmere."