The “starry-eyed, hard left” economic strategy of Jeremy Corbyn would hand the Tories at least another decade in power and end up hurting poor people by leading to higher inflation and interest rates as well as cuts in public spending, the shadow chancellor has said.

As Corbyn outlines plans to end “the years of political and economic austerity” to help create a high-skilled workforce in Britain, Chris Leslie has become one of the most senior Labour figures to say he would decline to serve under the veteran leftwinger.

In a sign of the deep unease at senior levels of the Labour party that Corbyn could be on the verge of a historic breakthrough by the left to win the party’s leadership, Leslie told BBC Radio 4’s Today programme on Monday: “This is a fork in the road for the Labour party. On 12 September we will know what the fate is of the progressive left of centre. There are millions of people whose living standards and working conditions depend on making sure we get this decision right, otherwise we face a decade or more of Conservative government.

“You can understand the frustrations of many in the Labour party. They want big bold clear solutions but it is the detail now we have got to get into. I am afraid some of those solutions, the policies offered on the hard left, are not all that they are cracked up to be. In fact they risk hurting some of the most poor, the most vulnerable, those on the lowest incomes.”

Leslie, who is backing the shadow home secretary, Yvette Cooper, in the leadership contest, cited Corbyn’s plans for a people’s quantitative easing and his pledge to deliver an extra £120bn in revenues through a crackdown on tax evasion and tax avoidance. He said: “Take this suggestion that there should be a people’s quantitative easing, in other words the Bank of England should be able to just turn on the printing presses and magically deal with all the public service and public investment needs that we have. At one level that sounds fantastically easy – if there is a shortage of money print some more. The difficulty is that if that then provokes higher inflation, if that then means that interest rates go up who will pay the price for that? It is the poorest and those on the lowest incomes who already find the cost of living very difficult.”

Leslie said he would decline “on principle” to serve in a Corbyn shadow cabinet because the party would lose economic credibility, which he said involved identifying the resources for public spending. “It is also about making sure you have that public support and consent for the collective provision of public services,” Leslie said. “If you don’t get that collective consent amongst taxpayers then those taxpayers will go for private health insurance or private education. You actually undermine the case for the public realm which is what we should all be about in the Labour party.”

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The shadow chancellor was also highly critical of Corbyn’s pledge to raise an extra £120bn in revenues through a tougher crackdown on tax avoidance and tax evasion. “Of course we have got to do more on tax avoidance and tax evasion. But you can’t just pluck a figure out of the air because otherwise you let people down who depend on decent, well thought through policies.”

In an interview with the Independent, Leslie warned that “Corbynomics” takes a “starry-eyed, hard left” approach. But Corbyn will brush aside warnings that he risks a return to the divisions of the 1980s when he pledges a return to the “sound economics of public investment”.

Corbyn is to unveil what his team is describing as his “Vision for Britain 2020”. The remaining weeks of the campaign will on his plan to end what he has called the “variations of austerity” from the main parties. A spokesperson for his campaign said: “This week Jeremy will continue with his strategy of focusing on policy. He is expected to further flesh out his economic vision for Britain 2020, signposting the way a Labour government under his leadership intends to return all of our United Kingdom to the sound economics of public investment so as to bring an end to the years of political and economic austerity.

“Jeremy’s Vision for Britain 2020 under his leadership is to promote economic growth and create a world-leading, high-skilled prosperous workforce and create a fairer Britain for all.”