He said that ordinary people would see the decision - after 11 years as leader and as he approaches his 58th birthday - as "essentially human.
"Indeed when I was first elected for Yeovil in 1983, I promised myself that I would not do the job beyond my 60th birthday and would stand down no later than the general election after that date. I believe that judgement was right," he said.
The soldier turned diplomat and politician has told friends he has no specific post lined up. But it is thought he believes he still has one more job left, although it is almost certain to be outside politics.
Whatever job he takes it will be one that allows him to spend more time with his family. In his letter to MPs, he wrote: "For Jane and myself... this will be a chance to rediscover parts of our life with our family and friends which have had to be pushed to the margins in these past hectic years." He is likely to withdraw almost completely from the Liberal Democrats, not wanting to undermine his successor.
Mr Ashdown wrote: "No doubt there will also be new challenges to seek out. I do not know what they will be yet. But, I do know that, love this calling though I do, I have never been so obsessive about politics that I see it as the only thing there is in my life!"
Some party sources were predicting last night that his future was in Europe where he has already won distinction by demonstrating a special understanding of the recent turmoil in the Balkan states and former Soviet Union.
The ideal role for Mr Ashdown might already have been created as the prestigious new post of EU Foreign Policy Tsar.
Senior diplomatic sources speculated last night that Mr Ashdown now stands a very strong chance of becoming Britain's nominee for the position. The job is being created in response to the long-held complaint that foreign heads of state never know who to ring at the EU when a crisis breaks.
The Government has already nominated Sir David Hannay, the former British ambassador to the UN. But EU leaders agreed last month that the post, known by its French acronym as Mr or Ms Pesc, must be given to an authoritative political figure rather than a career civil servant.
Mr Ashdown's immense energy and his passionate involvement in troubled spots such as Bosnia and Kosovo would enhance his chances, it is believed. Against that, he has never been a government minister and he is not yet backed by the two big European political "families", the Socialists and the Christian Democrats.
He would also be suited to a position with the United Nations or Nato.
Closer to home, Mr Ashdown's special interest is Northern Ireland. He served there in the Army, his family are from the province and he was teased at school for his Ulster accent. His links would attract him to a role working for peace.Reuse content