Long-standing Berwick MP Sir Alan Beith is to step down in 2015.
The Liberal Democrat will be then have served 42 years as an MP, taking the seat for the first time in 1973.
Sir Alan said: “I intend to continue to be a very active MP over the next two years and look forward to working closely with a candidate for 2015 who will be chosen by all the members of the Berwick constituency’s Liberal Democrat Association.
“This constituency has always been hotly contested between Liberals and Conservatives but it has been Liberals who have represented it for the majority of the years since the modern seat was created in 1885.
“Liberal Democrat commitments to freedom, fairness, representing local communities and helping people to get on in life fit well with the concerns of people in Northumberland.”
There will be events later this year to celebrate Sir Alan’s 40 years as Berwick’s longest-serving MP and the longest-serving Liberal MP since Lloyd George.