LEAVING a political party and joining another is not an easy thing to do. But what do you do as an M P if you become convinced that your own party has gone irretrievably off the rails, and another one really does now stand for the things you have always believed in?

There are only three things you can do in those circumstances. Leave public life altogether. Just suppress your honest judgement, and pretend to everyone that you are perfectly happy, which means mouthing falsehoods every time you make a political comment. Or “cross the floor” of the House.

I excluded the first and the second. To leave public life because circumstances had proved difficult would be an abdication.

To play dishonest games with the public was out of the question. It would have been a sustained and deliberate betrayal and deception of every one of my constituents.

I gradually realised that there was only one logical and remotely honest solution. The decision was painful and took months. Since I finally made it last weekend, of course I have worried about the shock and upset I would inevitably cause many supporters. Even publishing my detailed reasons (in my letter to David Cameron) would not entirely assuage that. But I knew that there was no other way.

Does this mean that I intend to fight the seat at the next election as a Labour candidate? No. That would create more conflicts of loyalty, more anguish and more emotion on the part of some of my longest-standing friends and supporters.

But of course it means that I shall continue to do the job for which I was elected to the end of my mandate.

That is how our Parliamentary system has always worked. And so long as I sit in the House of Commons I shall try to continue to live up to its best traditions, and to choose the best means available to advance the interests of all my constituents, and those of the country as a whole.

Full text of letter to David Cameron