A former Labour minister has resigned the whip in protest at the party's shift to the left under Jeremy Corbyn.
Lord Warner, a former health minister, quit the party warning that "I fear for Labour's future" if activists loyal to the leader gain greater control.
The Guardian reported that his resignation letter said the party could only hope to win another election with policies that win back voters who have switched to Ukip and the Tories.
A Labour source confirmed the resignation and said it "comes as no surprise" as Lord Warner had rebelled on a series of NHS-related measures in the last parliament, before Mr Corbyn took over as leader.
"It feels very ego-driven," the source claimed.
The source also said it was "a bit unfortunate that he didn't give anyone the opportunity to have a conversation with him about it".
Lord Warner, who served as a minister under Tony Blair from June 2003 to January 2007, said Labour was no longer "a credible party of government-in-waiting".
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."
He said he would continue to argue for progressive causes as a crossbencher in the House of Lords, but added 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 tweeted: "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."
The report, published in March last year, suggested that every resident would gain ''NHS membership'' for a monthly fee to be collected alongside council tax.
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