Ken Clarke has told ITV News that he has been "demob happy" for weeks at his decision to step down from the government, as he ended a front bench career stretching back to 1972.
The 74-year-old said he had enjoyed a "good-long innings" but now was the right time to step down from his role as Minister Without Portfolio.
Mr Clarke, who has held government posts under four different Conservative prime ministers, confessed that he had spent far more time at England's cricket Test Match against India last week than in his office.
"I thought I'd do a couple of years in this government. That was the agreement I had with David [Cameron]. I've done four and that's obviously enough, and it makes sense to have a reshuffle," Mr Clarke told ITV News.
Mr Clarke admitted he was a "life-long addict to politics" was "as keen as ever" to carry on as an MP for Rushcliffe, his Nottingham constituency.
He promised to be "largely supportive" from the backbenches.
This is the full letter written by Ken Clarke to David Cameron announcing his retirement from government:
I understand that you are proposing to re-shuffle the Government this week and I am writing to let you know that I would wish to retire from Ministerial office in your Government. I have greatly enjoyed my four years in your Cabinet and my spell in the National Security Council. I have been heavily engaged in some parts of my portfolio such as the EU/US Trade Agreement, economic reform in Europe, support for small business, export finance, the fight against corruption, secure hearings in Courts and so on. However, I have just celebrated my 74th birthday and I have been doing red boxes at night for a high proportion of my adult life. There are plenty of other able people who could take on the work that I was doing in Government and I think the time has come to return to being a veteran back bencher.
It has been fascinating to be in a Government that has had to face the worst financial crisis of my lifetime, and the aftermath of the deepest and longest recession since the war. I believe that we have saved the country from economic disaster, although we have a long way to go before we have the modern, healthy and competitive economy that the next generation needs. I have always been a radical reformer of public services and I think that people will look back on our achievements in modernising the Health Service and Education Service and the Welfare system in a favourable way. I also had the chance of introducing a truly radical and reforming Criminal Justice Bill which I think went some way toward paving the way for a system whereby punishment of serious criminals can properly be combined with the reform of rehabilitation to stop many of them offending again.
I intend to remain as an active back bencher in the House of Commons. My belief in Britain's membership of the European Union remains as firm as ever and I think the political and economic case is made even stronger in today's globalised economy and dangerously disturbed world. We must not diminish Britain's ability to influence events in the next few decades. I know that you are quite determined to have a referendum on the subject, in which I will be campaigning vigorously for a vote to keep us in the Union.
I wish you and all my colleagues well in their continuing efforts and will work for a Conservative success in the next Election. I will still no doubt occasionally offer you the benefit of my advice on various subjects, either from the back benches of the House of Commons or in the occasional private meeting.
Thank you very much for the privilege of serving in your Cabinet for the last four years.