Lord Grabiner has become the second Labour peer to leave the party whip in a week.
In a letter to The Times this morning, he said he left at the end of September due to his differences with Labour leader Jeremy Corbyn.
“I can't square staying with my conscience,” he said. “I have nothing in common whatever with Corbyn — and I don't believe we are ever going to win an election."
I was a David Miliband man but we then had five years of Ed Miliband and now Corbyn — that’s my real problem.
He also voiced concerns about the appointment of John McDonnell as Labour chancellor: "I am concerned with the economic stuff; I am really concerned with the shadow chancellor."
The QC and master of Clare College Cambridge joined the House of Lords 16 years ago. He will remain a member of the party, but will instead be a crossbencher.
His announcement makes him the second Labour peer to publicly resign in a week. On Monday, former health minister Lord Warner quit, on the grounds that Labour was no longer “a credible party of government-in-waiting”.
In a letter to Corbyn, who was elected as party leader last month, Warner said:
Labour will only win another election with a policy approach that wins back people who have moved to voting Conservative and Ukip, as well as to Greens and SNP. Your approach is unlikely to achieve this shift.