The Labour party has always been a broad church whose internal disagreements have given it much of its vitality and relevance. It is impossible for MPs to sit in the Commons for years without ever disagreeing with their leaders. What varies is the individual’s willingness to show public dissent, to defy party orthodoxy and party whips. Frank Field, MP for Birkenhead since 1979, is one who has not been shy of taking an independent line. Another is Jeremy Corbyn, MP for Islington North since 1983.

For 35 years, Mr Field and Mr Corbyn have sat on the same side, regularly disagreeing with their party’s policy and with each other. Neither felt obliged to resign the whip until now. Mr Field on Thursday declared his intention to sit in the Commons as an independent. That is all the more extraordinary since, three years ago, he was one of the MPs whose nominations secured Mr Corbyn’s place in the contest that saw him elected as leader.

Mr Field said at the time that he was pleased with his choice, because the other candidates were offering thin “post-Blair gruel”. On Thursday, he accused Mr Corbyn of acting as “a force for antisemitism” – by refusing to recognise how comments he had made in the past emboldened anti-Jewish prejudice – and of tolerating a wider culture of “intolerance, nastiness and intimidation”. In levelling these charges, Mr Field drew on experience in his local constituency party. He was not inventing the factional aggressions, which makes it imperative that his concerns be taken seriously.

The inappropriate response – sadly, one that some of Mr Corbyn’s supporters leapt for – is to suggest that Mr Field is an embittered has-been who was facing deselection anyway, so an unreliable critic of the leader. It is also wrong to imagine that he represents some wider pattern of anti-Corbyn sabotage. There are Labour MPs who crave a change of leader but they are disorganised and weak. Also, on the single biggest issue of the day – Brexit – most anti-Corbyn MPs are in fundamental disagreement with Mr Field. They tend to be staunchly remain; he is an eager leaver. There is no conspiracy here, just frustration and dismay.

Mr Field’s support for Brexit is not the only way in which he has offended Labour members. He has taken positions on welfare reform (on the relationship between benefits, culture and incentives to work, for example) usually expressed by the Conservatives. After the 2010 election, he led a commission on poverty, authorised by the coalition government. That association with David Cameron is contamination enough in the eyes of some left purists.

But Mr Field has also been a powerful critic of Tory policy. No one who has followed his career, including his role since 2015 as chair of the Commons work and pensions committee, can doubt his commitment to fighting poverty. Nor are there grounds to doubt his integrity when it comes to criticising leaders, past and present. His reasons for resigning the whip are the ones he has given, and they should be taken seriously. It would be unwise to dismiss the judgment of someone who, in many years as an independent spirit, has never before felt so alienated by things that have been said by a Labour leader and done in the leader’s name.

If Labour is not a broad enough church to accommodate Mr Field’s creed, if respected veterans and those who share his concerns feel their presence is unwelcome, the party will be politically diminished and much less able to protect the weak, whom Mr Field and Mr Corbyn have in their different ways struggled all their political lives to defend.