Clive Lewis, the Labour shadow minister and former leadership candidate, is to urge those still in the race to embrace electoral reform, arguing that there are overwhelming moral and political reasons to ditch the first-past-the-post system.

In speech on Tuesday night to a meeting of the Labour Campaign for Electoral Reform, Lewis is expected to bemoan the fact that the party regularly flirted with the idea of proportional representation, only to ditch it when an election win seemed possible.

“The narrative goes something like this: ‘Labour can’t win under first past the post – so we have to change the system,’” the Norwich South MP is expected to say in the speech in parliament, according to extracts released in advance.

“This blip usually lasts as long as its takes to get to the incumbent government’s mid-term doldrums and the race to the next electoral finishing lines comes in to view. At which point you have to pretend you can win alone – even when its obvious you can’t. Support for PR then sinks until Labour does lose again. Play and repeat, ad infinitum.”

Lewis, a shadow Treasury minister, argued for his party to embrace PR as part of his pitch to replace Jeremy Corbyn. He did not advance beyond the first stage of the race after failing to win enough nominations from fellow Labour MPs or MEPs.

In the speech, Lewis will say the arguments for change can be summed up as an ”alliterative triptych of PR, which is – maths, morality and modernity”.

Under first past the post, he is to argue, while it takes an average of 50,000 votes to elect each MP at the last general election, that varied from 38,000 for every Conservative representative, 336,000 per Lib Dem and 865,000 for the Greens’ sole MP.

Lewis says Labour has resisted electoral reform because “the desire for so-called strong government combines with a deep sense of tribalism that sees anyone outside of Labour’s ranks, and indeed many people within Labour, as the enemy”.

This was wrong, he is to say: “We back electoral reform not because it might be good for us but because it’s good for all of us. We back it because we believe in the equal and powerful voice of all. We hate wasted votes just as we hate wasted lives. We back it because we are democrats – unlocking all the voices.”

First past the post is also anomalous in an increasingly complicated and interconnected world, Lewis will say

“It is almost laughable that we think climate change, our ageing society, the rise of the robots, the epidemic of loneliness and so much more can be outsourced to a government that wins power on the basis of 25% of eligible voters. That’s not a mandate – it’s a shackle.”

Lewis will say he is “listening really hard to hear a candidate who really gets this, who doesn’t just back PR when pushed to but wants to make it a central issue to their leadership and really champion it”.

Keir Starmer, the frontrunner to be the new leader, has called for a constitutional convention to address issues including “the fact that millions of people vote in safe seats and they feel their voice doesn’t count”.

Among the other candidates, Rebecca Long-Bailey has also backed a constitutional convention without explicitly backing PR, while Emily Thornberry opposes the idea, arguing against moving to a system that is not based around constituency MPs. Lisa Nandy has supported PR in some ways, but has not backed arguing for it in general elections.