A former Labour minister has resigned the whip in protest at the party's shift to the left under Jeremy Corbyn, claiming it was no longer a "credible" alternative government.
Lord Warner, a former health minister, quit with a warning that "I fear for Labour's future" if activists loyal to the leader gain greater control.
The peer's decision triggered a furious response from Labour politicians, with one MP calling him an "arse" and former deputy prime minister Lord Prescott saying he was "no great loss".
Lord Warner, who served as a minister under Tony Blair from June 2003 to January 2007, said Labour's direction under Mr Corbyn would "worsen the decline" in the party's fortunes.
The Guardian reported that his resignation letter to Mr Corbyn 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."
Lord Warner said he would continue to argue for progressive causes as a crossbencher in the House of Lords, but he did not think "those are likely to be those you or your kindred spirits espouse".
He wrote: "I have watched for some time the declining quality of the Labour Party's leadership, but had not expected the calamitous decline achieved in 2015.
"The Labour Party is no longer a credible party of government-in-waiting. The approach of those around you and your own approach and policies is highly likely to to worsen the decline and in the Labour Party's credibility.
"I fear for the future of the Labour Party if your supporting activists secure ever control of the party's apparatus and process, and the role of the parliamentary Labour Party diminishes further in the selection of a leader and the formulation of policies likely to win an election."
Lord Warner's policy positions have often been at odds with his party, including co-authoring a report that suggested people should be charged a £10 monthly membership fee for using the NHS alongside hotel-style charges for hospital stays.
Former deputy prime minister Lord Prescott wrote on Twitter: "Lord Warner. A minister you've never heard of who wanted to charge people £10 a month to use the NHS. No credibility. No great loss."
Shadow work and pensions secretary Owen Smith told BBC2's Newsnight: "I think Lord Warner has been leaving us for quite a while. He voted with the Tories a couple of years, under the last Labour leadership, to privatise parts of the NHS. He is somebody who has advocated charging for the NHS, charging to stay overnight.
"I think he has been leaving Labour for quite a while. I'm not sure we will miss him too much."
Labour former minister John Spellar simply said: "He was always an arse."
A Labour source said the resignation "comes as no surprise" and "feels very ego-driven".