Commenting, Liberal Democrat Leader and Deputy Prime Minister, Nick Clegg said:
"Sir Alan has been a huge figure in British and both Liberal and Liberal Democrat politics over an astonishing four decades.
"The present generation of political leaders in Britain were barely out of nursery when Alan Beith was first making his mark on the floor of the House of Commons.
"So it is little wonder that when Sir Alan speaks today, he commands the attention of all parties in Westminster.
"Sir Alan has held and excelled at pretty much every key role in the Liberals and Liberal Democrats - from Chief Whip to Deputy Leader and from Home Affairs spokesman to now Chair of both the Justice Select Committee and the Liaison Committee.
"Speaking personally, I have learned to trust and rely on Alan's enormous wisdom and to greatly admire his passionate commitment to his Berwick upon Tweed constituency and to the North East, which shines through in everything he does."
Announcing his intention to step down in 2015, Sir Alan Beith 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."
The Facts
Members of Parliament for the Berwick upon Tweed constituency since the Redistribution of Seats Act 1885, when the constituency became a single member constituency:
1973 - present Alan Beith, Liberal Democrat
1951-1973 Antony Lambton, Conservative
1945-1951 Robert Thorp, Conservative
1944-1945 William Beveridge, Liberal
1941-1944 George Grey, Liberal
1935-1941 Hugh Seely, Liberal
1929-1935 Alfred Todd, Conservative
1923-1929 Mabel Philipson, Conservative
1922-1923 Hilton Philipson, National Liberal
1916-1922 Francis Blake, Liberal
1885-1916 Edward Grey, Liberal
Sir Alan Beith has today announced his intention to stand down as an MP in 2015 after four decades representing Berwick upon Tweed. He is the longest serving Liberal/Liberal Democrat MP since Lloyd George and currently chairs the parliamentary Justice Committee and Liaison Committee.