Andy Burnham has reignited hostilities with Keir Starmer’s Labour leadership, criticising what he described as the “bankruptcy” of the party’s approach to campaigning, a week after it lost the previously safe seat of Gorton and Denton.

The mayor of Greater Manchester and former MP, regarded as a rival to Starmer, said Labour’s campaigning style prevented it from connecting with non-Labour voters and other progressive parties, as he evoked the system of clipboard-wielding canvassers going door to door with records of previous Labour supporters.

“What I want to say today is that the time has most definitely come for a serious conversation about our political system and its pervading culture, particularly so in the aftermath of the Gorton and Denton byelection,” Burnham said in a speech at the British Library in London that reignited speculation he has not given up on replacing Starmer.

“It revealed the full depth of the chasm between people and Westminster politics. I don’t think anybody can seriously dispute that statement.”

Burnham was speaking a week on from Labour’s loss of its once safe seat in the Manchester constituency, after Starmer and his allies blocked him from standing to be the party’s candidate.

Labour’s deputy leader and Burnham ally, Lucy Powell, has said he would have won the contest, in which the Green arty’s candidate, Hannah Spencer, was victorious. Labour came third, with Reform UK in second.

Burnham described polling by More in Common that found a majority of people did not think the cost of living crisis would ever end as “code red for Westminster politics”.

“This is getting extremely dangerous, and change in our political system and culture is desperately needed,” he added.

The mayor answered a number of questions after the speech, which took place at an event organised by the Centre for Cities, but remained silent when Andrew Carter, the thinktank’s chief executive, said that a question about allegations of so-called “family voting” irregularities did not fall under the event’s “rules”.

In his speech, Burnham launched a withering attack on his colleagues in the UK government, claiming that Westminster and Whitehall no longer appeared to want to “share growth” with regions such as the north of England.

He also prompted laughter from the audience when he said he wanted to turn Manchester into Britain’s “leading green city”, before adding: “Some might say it took a step that way recently.”

Burnham said he wanted to use the speech to lay out for the first time in detail his vision of what he described as “Manchesterism” – a way of governing that has become associated with the former MP’s apparent pitch for the Labour leadership but which he identified as “the opposite of Westminsterism”.

However, he also projected a frustration with Whitehall, railing against what he said was “the resistance of the system to free us up more”.

“After 10 years of devolution they are still pushing us away as if they know all the answers, and still they hold on and refuse to devolve,” he told the audience

“I am getting to the point where I refuse to spend more of my time making the case. It just makes you think they don’t actually want growth everywhere. They just want to hold on to things down here. We need Whitehall reform but we also need Westminster reform.”