REUTERS/Suzanne Plunkett
As well as shaping the future of British economic policy, the vote is a crucial test of Labour leader Jeremy Corbyn's control over his party.
Labour shadow chancellor John McDonnell initially supported Chancellor George Osborne's new budget law. But on Tuesday he said the party would oppose it, throwing Labour into disarray.
Many within the party believe Labour should support the new law to help rehabilitate the party's economic image. And confusion within the party wasn't helped by the fact that even Jeremy Corbyn was apparently unaware of the shock change in policy coming from McDonnell, according to the BBC's political editor Laura Kuenssberg.
All sorts of reasons why McDonnell U turn on budget surplus not a laughing matter, including that Corbyn himself had no idea it was coming
- Laura Kuenssberg (@bbclaurak) October 13, 2015
A fortnight ago, Labour told voters they were ready to back our plans. But now, they have confirmed they want to go on borrowing forever - loading debts onto our children that they can never hope to repay. This is not socialist compassion - it's economic cruelty. As Labour's Great Recession showed, those who suffer most when government run unsustainable deficits are not the richest but the poorest.
So today, with Labour's economic policy in obvious chaos, I call on all moderate, progressive Labour MPs to defy their leadership and join with us to vote for economic sanity. Failing that, they should at least follow the advice of the former shadow chancellor and abstain.
Five party frontbenchers resigned after Corbyn was elected, including former shadow health minister Jamie Reed who tweeted his resignation during Corbyn's acceptance speech.
Jeremy Corbyn has struggled to shift the image of him as a "radical" left-winger put forward in the press and by opponents and these Labour MPs find Corbyn's ideas too left-wing.
Osborne is doing his best to exploit and emphasise this image of Corbyn in order to pull the rug out from underneath him and coax a few MPs to the other side of Parliament's dispatch box.
The most prominent scalp so far has been Lord Adonis, the former policy chief under Labour Prime Minister Tony Blair, who announced he would head a new government body that will plan infrastructure projects earlier this month. While Lord Adonis remains a member of Labour Party, he has resigned the party whip meaning the leadership can't tell him which way to vote.
Osborne is also going after centre-ground voters who may have been put off by Labour's move to the left. In his party conference speech the Chancellor told Conservatives to "extend our hand" to people who feel "completely abandoned" by Labour, and went so far as to say the Tories were "the true party of labour."
In response, former Labour Party and GMB trade union official turned Telegraph columnist Dan Hodges declared David Cameron to be the "leader of the British left."
David Cameron is now the leader of the British left.
- Dan Hodges (@DPJHodges) October 7, 2015
I'm not going to join the Tories. But it may take me a while to come up with a coherent reason why.
- Dan Hodges (@DPJHodges) October 7, 2015