Rachel Reeves will warn the Reform UK "clown show" would lose control of the public finances and heap misery on ordinary people.
The Chancellor is expected to launch an attack on Nigel Farage's economic plans, which experts have previously said fail to add up.
It comes as Ms Reeves is expected to address Cabinet on Tuesday on plans to drive up growth ahead of a highly-anticipated Budget in November.
She is expected to tell MPs: "This Labour Government is building an economy that works for working people – and rewards working people.
"That means cutting the deficit, getting debt falling and investing responsibly in Britain’s future. That is in sharp contrast to the Conservative Party that is still in denial at the damage done by Liz Truss.
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"And the clown show we witnessed last weekend, with a Reform Party that would lose control of spending and force up inflation and people’s mortgages."
Last year, Reform proposed a £140billion spending spree, including £88billion in tax cuts - more than double the £45billion Liz Truss announced in her disastrous mini Budget.
The plans were to be paid for by £156billion of savings in public spending, including abolishing net zero commitments and slashing Government waste.
At the time, the Institute for Fiscal Studies think tank said the sums "do not add up" and "would almost certainly require substantial cuts to the quantity or quality of public services”.
Reform's Richard Tice said last week: "The manifesto in July 2024 is not appropriate for a manifesto, or a contract, whenever the next general election is." But it offers the clearest indication so far of their thinking.
It comes as the head of the Trades Union Congress branded Mr Farage and pal Mr Tice "right-wing con men lining their own pockets".
TUC General Secretary Paul Nowak accused Reform UK of "selling out" ordinary people by opposing workers rights reforms and backing Donald Trump whose tariffs threaten British industry.
He told delegates in Brighton: "There is a world of difference between what Nigel says and what Nigel does.
"Every single Reform MP, including Mr Farage, voted against a ban on fire and rehire, against banning zero hours contracts and against day one rights for millions of workers.
"Here's my challenge to Nigel Farage. Say you stand up for working people? Then ignore your wealthy backers and vote for that Employment Rights Bill. Say you stand up for British industry? Then stop supporting Donald Trump and his destructive tariffs."
He said it was time for Mr Farage to "come clean" about whose side he is on, adding: "Here's the truth - you're not representing working people, you are selling them out."
Mr Nowak went on: "To those who voted Reform at the last election, and to those who are considering voting for Reform, I get your frustration with mainstream politics.
"I get your sense that change isn't coming fast enough and I respect your right to vote for whoever you choose.
"But ask yourself this fundamental question - do you believe, in your gut, that Nigel Farage really cares about the people of Clacton, when he is off collecting his speaker fees in the United States?
"That Richard Tice really worries about the people of Skegness, while he's living it up at home in Dubai? Or are they just right-wing con-men, lining their own pockets?"

Mr Nowak also called on Keir Starmer to “show working class communities whose side you are on” by standing firm on worker rights reforms.
Some union leaders are nervous that the Employment Rights Bill could be watered down when it comes back to Parliament next week.
The flagship reforms will deliver day one rights to workers and ban bad bosses from employing workers on exploitative zero hours contracts, or firing and rehiring their staff on worse terms.
Angela Rayner's resignation last week, and the sacking of Justin Madders, regarded as one of the architects of the plans, has fuelled concerns.
The TUC leader warned that Labour was elected on a manifesto promising change, but for too many people, "change feels like a slogan".
He said: "My message to the Government is simply this - deliver the manifesto on which you won a huge majority last July. Deliver good jobs, decent public services and better living standards in every corner of the country.
"Deliver the change people voted for and show working-class communities whose side you are on."
Housing Minister Alison McGovern insisted the Government was committed to the plan, which was a "cornerstone" of Labour's election manifesto. She told a fringe meeting at the TUC Congress in Brighton: "Labour MPs have voted three times for it. We are committed to it."
Downing Street insisted the Bill would not be watered down - and the Government would overturn amendments made by Lib Dem and Tory peers next week.
The PM's official spokesman said: "We are absolutely backing the Employment Rights Bill. We are a pro-worker, pro-business Government and the workers' rights legislation is the biggest single upgrade of workers' rights in a generation."
On Monday evening, Keir Starmer told MPs the Employment Rights Bill “will continue with the same substance and timetable as before”.
He also branded Reform UK “plastic patriots” as he addressed Labour MPs in Parliament. In a meeting with the Parliamentary Labour Party, the PM criticised those whose “politics are grounded in grievance”. He spoke about Nigel Farage’s trip to the US, saying he called for “sanctions against our country that would hit working people”.
During the meeting, the PM called Reform “plastic patriots”. He also said there are “plastic progressives on the other side too”.
He said: "There's nothing progressive about a Green Party that opposes green infrastructure, blocks house building and wants to take us out of NATO."
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