Lord Hacking, who has been a Tory hereditary peer since 1971, says in a letter to The Independent today that his move has been prompted by changes in Conservative and Labour policies on penal affairs and Europe.
"The choice is party or policy," Lord Hacking, 60, says in a moving letter that will strike a chord with many Conservatives. "Difficult though it is, I believe the only honest choice is one of policy."
With the Prime Minister opening the final summit of his European presidency in Cardiff today, the latest Tory scalp will come as welcome evidence of Labour's positive attitude towards the European Union - and William Hague's steady drift into Euro-scepticism.
Lord Hacking, a partner in the London law firm Sonnenscheins, says that since the Tories lost office last year, they have been turning away from established policy towards the EU.
"It was my party who negotiated our entry into the EEC. It was my party who negotiated and signed the Single European Act in which we expressly agreed the convergence policies for bringing into being the single currency. It was my party who negotiated and signed the Maastricht treaty in which we expressly agreed the establishment of economic and monetary union, including the single currency ..."
Now, he says, Mr Hague talks of a single currency as "the economic equivalent of a burning building with no exits". Far from seeking to be at the heart of Europe, he adds, "we are now seeking to be apart from it".
Lord Hacking has similar disquiet about the direction of Tory policy on law and order. "Much has changed in the Conservative Party - as it has in the Labour Party - during my adult life," he says. "Recent changes in my party have not been to the good. At the end of its period of office, just over a year ago, we had a Conservative government which was standing on its head its own established penal policy. Claims by the then Home Secretary that `prison works' and the introduction of measures which imposed mandatory sentencing on the judiciary were populist and unworthy."
Examining the dilemma faced by all politicians in such circumstances, Lord Hacking says he can either stay with the Tories and try to work for a restoration of old commitments, or join Labour, already committed to a strong but compassionate penal policy and a determination to make the EU work from within.
"On Monday," he says, "I will be `crossing over' the floor of the Chamber of the House of Lords from the Conservative benches to the Labour benches."
As with all those who have made the same move in recent years, Lord Hacking says he leaves in sorrow. Having belonged to the Conservative Party for the past 40 years, he will be leaving friends behind. "I realise I will be causing upset," he says. But the growing frequency of the act measures the change in both parties.
Letters, Review, page 2