Norman Warner, a Labour health minister between 2003 and 2007, became the first Jeremy Corbyn-era Labour parliamentarian to resign the whip, saying Labour is no longer “a credible party of government-in-waiting”.

In a letter to Corbyn, Lord Warner said he would sit as a cross bencher in future, adding: “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 on the Lords benches, but he does not believe “those are likely to be those you or your kindred spirits espouse”.

He writes to Corbyn: “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.”

Warner was instrumental in pushing through many of Tony Blair’s health reforms, but was also a key adviser to Jack Straw as home secretary.

The Corbyn team may dismiss his resignation, but they will be wary of saying anything that alienates other Labour peers, partly because Warner is widely respected as an assiduous peer and expert on health care reform. Few Labour peers have any sympathy for Corbyn and unlike Labour MPs are not subject to his patronage.

In his resignation letter, Warner quotes the late Labour chancellor Denis Healey, who said in a 1959 speech to the Labour party conference after the party’s third successive general election loss : “There are are far too many people who want to luxuriate complacently in the moral righteousness of opposition – we are not just a debating society.

“We are not just a Sunday socialist school. We are a great movement that wants to help real people living on this earth at the present time. We shall never be able to help them unless we get power.”

Owen Smith, Labour’s Shadow Secretary of State for Work and Pensions, said that Lord Warner had been leaving the party for “quite a while” and would not be missed too much.

“He voted with the Tories a couple of years ago under the last Labour leadership to privatise parts of the NHS,” he said on BBC’s Newsnight.

“He’s someone who has advocated charging for the NHS, charging to stay overnight. I think he’s been leaving Labour for quite a while. I’m not sure we will miss him too much.”