Image copyright PA Image caption Mr Leslie said working people would suffer, not prosper, under a Corbyn leadership

Shadow Chancellor Chris Leslie has said he would not serve under Jeremy Corbyn if he becomes Labour leader.

He told the BBC Labour would "not be the party I joined" if Mr Corbyn won in September and the party needed a "credible" alternative prime minister.

Mr Corbyn, who has the most nominations from constituency Labour parties, is to detail more of his policies this week.

He will reportedly spell out a plan to end "political and economic austerity" by 2020 through higher public spending.

The Guardian said Mr Corbyn was expected to elaborate on his plans for a fairer Britain and a more highly-skilled workforce through the "sound economics of public investment".

The leadership contest - in which Andy Burnham, Yvette Cooper and Liz Kendall are also standing - has been turned on its head by the performance of Mr Corbyn, the veteran left-wing MP who only got enough nominations from MPs after a number who did not back him decided to nominate him to "broaden debate" during the contest.

He has topped two early opinion polls and won the backing of four major unions but his growing popularity has prompted a backlash from senior figures in the party, ranging from Tony Blair to Lord Kinnock.

Mr Leslie, who became shadow chancellor in the wake of May's election defeat, said the leadership contest was a "fork in the road" for the opposition and that the party faced a decade out of power if Mr Corbyn prevailed.

Analysis - by assistant political editor Norman Smith

If this was meant to be the start of a concerted fight-back by senior figures in the Labour Party against Jeremy Corbyn it seems to have fizzled out in a morning - and it reflects a shadow cabinet in a state of confusion.

Some shadow ministers take the view that any attacks on Jeremy Corbyn fuel his argument that he is the anti-establishment anti-business-as-usual candidate. Those around Chris Leslie take the view this is a profound moment in the Labour Party with an ideological decision to be taken on whether it becomes a quasi-Marxist organisation or remains a social democratic progressive force.

And then there is the bulk of the shadow cabinet, who seem to be not quite sure what on earth they should do. If Jeremy Corbyn wins some say they would serve under him, others would not. It is a scene of complete and utter chaos.

Mr Leslie said there was a yearning within Labour for "big, bold" solutions but Mr Corbyn's policy of a so-called "people's quantitative easing" risked creating higher inflation and interest rates, placing a financial burden on poorer people.

"Some of the solutions offered on the hard-left are not all that they are cracked up to be," he told BBC Radio 4's Today programme. "In fact, they risk hurting some of the most poor, vulnerable and those on the lowest incomes."

Economic credibility "wasn't just about winning elections", he said, but "making sure you can say where the money is coming from for those better public services".

Mr Leslie, who was closely involved in shaping Labour's economic policy under Ed Miliband, said "on principle" he did not think he could serve in a shadow cabinet under Mr Corbyn although he vowed to remain in the party.

"It would be a very different political party (if Mr Corbyn won)," he argued. "It would not be the party I joined."

Fellow Labour MP John Woodcock, who is backing Liz Kendall, told BBC Radio 4's World at One he was concerned about the party's image "taking a massive leap back to the politics of Michael Foot and the 1980s" following the surge in support for Mr Corbyn.

But Richard Murphy, an economic adviser to Mr Corbyn's campaign and of Tax Research UK, said Mr Leslie has "got this completely wrong" as he defended the plan for "people's quantitative easing".

He told World at One: "The real question for Chris Leslie is 'why did you support £375bn for the banks?' when actually very much less would create jobs in every constituency throughout the UK, which is precisely what Jeremy Corbyn is offering by adopting this programme."

Mr Corbyn has said he will reintroduce elections for the shadow cabinet, which were scrapped by Mr Miliband. This means all MPs will have to decide whether to put themselves forward.

Ballot papers for the leadership election will be sent out on 14 August, with the result announced on 12 September.