Here is the full text of Sir Menzies Campbell's letter of resignation as Leader of the Liberal Democrats:
When I was elected Leader of the Party in March 2006 I had three objectives.
First, to restore stability and purpose in the party following my predecessor's resignation and the leadership campaign itself, second to make the internal operations of the party more professional, and third to prepare the party for a General Election.
With the help of others, I believe that I have fulfilled these objectives, although I am convinced that the internal structures of the party need radical revision if we are to compete effectively against Labour and the Conservatives.
But it has become clear that following the Prime Minister's decision not to hold an election, questions about leadership are getting in the way of further progress by the party.
Accordingly I now submit my resignation as Leader with immediate effect.
I do not intend to hold a press conference or to make any further comment.