Millions of families on low and middle incomes face a triple tax hit from Jeremy Corbyn.

Labour has repeatedly vowed that its £83billion-a-year tax proposals will affect only the rich and big corporations.

But the small print of the party's manifesto reveals that ordinary families, shareholders and small business owners could be hammered.

The first blow comes from Labour's plan to scrap the marriage tax allowance, which is worth £250 a year to more than 1.7million couples.

Millions of families on low and middle incomes face a triple tax hit from Jeremy Corbyn

Labour Party leader Jeremy Corbyn addresses the audience during a climate emergency rally in Falmouth on Wednesday

Secondly, a £9billion levy on dividend payments could hit individuals with modest share portfolios or small businesses. Lastly, a hike in capital gains tax – which Labour hopes will raise another £9billion a year – could punish those selling homes or shares.

The revelations came as a poll which accurately predicted the result of the last election forecast that Boris Johnson would win a 68-seat majority next month. The YouGov MRP survey put the Tories on 359 seats, Labour on 211, the SNP on 43 and Lib Dems on 13.

Former Tory leader Iain Duncan Smith, one of the architects of marriage allowance, attacked Labour's plans to scrap it. He said: 'In getting rid of it they are attacking millions of people often on very low incomes. You cannot get the allowance if you are on more than £50,000.

'Corbyn has also made it clear he is going after people who have built up small nest eggs in shares, and economists are very clear that Labour's plans to hammer corporation tax will lead to higher prices, which will hit the least well-off the hardest.'

Mr Corbyn was forced to admit on Tuesday – in a disastrous BBC interview with Andrew Neil – that his plans would hit families earning less than £80,000.

On another difficult day for Labour:

Boris Johnson's chief adviser Dominic Cummings said the election race was 'much tighter' than polls suggested – and warned Labour would 'cheat' Brexit supporters in any second referendum;

The Conservatives accused Mr Corbyn of 'out and out lying' after he produced leaked documents purporting to show the Government had discussed the NHS during trade talks with the US;

Labour is braced for more criticism of its tax and spending plans when the Institute of Fiscal Studies reveals a detailed analysis of its manifesto this morning;

Mr Corbyn was isolated over anti-Semitism as a string of his closest allies, including Shadow Chancellor John McDonnell, voiced regret at the party's failure to deal with the crisis;

Mr Johnson signalled a break with the austerity of the last decade, telling the Spectator magazine he had warned David Cameron in 2010 it was 'not the right way forward';

Nicola Sturgeon raised her price for a coalition deal with Mr Corbyn, saying she wanted an extra £4billion for the Scottish NHS, a second independence referendum and scrapping of Trident.

In Tuesday's interview, Mr Corbyn insisted lower earners would not lose out overall from Labour's plans because 'they will also be getting a pay rise when we bring in a living wage'.

Richard Burgon, Labour's justice spokesman, yesterday acknowledged that some lower-paid families could lose out but insisted 'all of these people will be better off' overall as a result of Labour's spending proposals.

Richard Burgon, Labour's justice spokesman, (pictured on Sky News today) acknowledged that some lower paid families could lose out but insisted 'all of these people will be better off' overall as a result of Labour's spending proposals

Under Labour's dividend tax plans, the tax-free allowance of £2,000 will be scrapped and basic rate taxpayers will see their bill rise from 7.5 to 20 per cent. Anyone with investments outside an ISA or pension could be hit by changes to dividend and capital gains tax.

Experts said pensioners on low incomes who are using investments to supplement their retirement could end up worse off. Former Tory pensions minister Baroness Altmann said: 'This isn't just rich shareholders, this is about ordinary people who have saved for their retirement.' Laura Suter, personal finance analyst at AJ Bell, said: 'Despite Corbyn's claim he would only raise tax for the wealthy earning more than £80,000, the scrapping of the marriage allowance and changes to dividend and capital gains tax will hit lower earners too.'

Married couples 'to lose £1,1000'

More than two million married couples could lose a tax break worth up to £1,149 under Labour's plans, it emerged last night.

The party's manifesto outlined proposals to scrap the marriage tax allowance as part of its measures to raise an extra £83billion a year in taxes.

The tax break, worth £250 a year, is currently claimed by about 1.8million couples. But official figures suggest around 4.2million are eligible.

At present, they are able to claw back payments for up to five years. Laura Suter, personal finance analyst at AJ Bell, said: 'Almost 1.8million people claimed the marriage allowance last year, and it will cost each of those couples up to £250 each a year if the tax break is scrapped. However, the potential hit is far higher for couples who are eligible for the break but have failed to claim it, if they can no longer backdate their claims.

'At the moment, couples can claim up to five years' worth of the allowance, which is worth a maximum of £1,149 per couple.'

Former work and pensions secretary Iain Duncan Smith, who was one of the architects of the tax break, said Labour's plans breached Jeremy Corbyn's pledge not to increase tax on those earning under £80,000 – because it was only available to those earning less than £50,000. Mr Duncan Smith said: 'Labour's claim to be only hitting the rich is complete rubbish and this is a prime example.

'It is also an attack on the family, including some of the poorest families in Britain. But Labour as a party doesn't believe there is any importance in marriage, despite all the evidence to the contrary.'

Former Liberal Democrat pensions minister Steve Webb said those claiming marriage tax allowance came 'a long way down the income scale' and included 'plenty of pensioners'.

Mr Webb, director of policy at Royal London, warned that many savers with share portfolios could also be hit by Labour's plans to push up dividend tax.

Labour has said it will axe the £2,000 tax-free allowance for dividend payments and increase rates in line with income tax. A basic rate taxpayer with dividend income of £2,000, who currently pays nothing, would be hit with a £400 bill.

Mr Webb said: 'There will be people well outside the top five per cent of earners who hold their shares outside an ISA for whatever reason who are suddenly going to find themselves facing a nasty tax bill.'

Similar concerns have also been raised about capital gains tax, where the tax-free allowance has been axed and rates are, again, being brought in line with income tax.

And there was a warning that small business owners, who often pay themselves in whole or in part via dividends, could also be hammered.

Mike Cherry, of the Federation of Small Businesses, said: 'These dividend proposals would mean a huge rise in tax payments that would threaten small business owners earning relatively modest amounts. On the face of it, a small business owner making £40,000 could face thousands of pounds more tax every year. This is a huge concern and urgent clarification is needed.'

Edwin Morgan, of the Institute of Directors, said 'many businesses' would be affected by Labour's dividends plans – which are designed to raise an extra £9billion.

Mr Morgan added: 'Dividends are paid after corporation tax, so this is not tax-free money.

'It is another burden on top of the burdens small businesses are already facing from things like a higher national minimum wage and pensions auto-enrolment.

'This is not a reform, it is a revenue raiser. And the economy is not going well enough to do that.'

Mr Corbyn insists that families that lose out from the axeing of the marriage tax allowance would gain from other Labour's proposals. He added: 'We have to recognise that we have to do something about the underfunding of our public services.'

Q&A

Why have Labour's tax plans flared up as an issue?

Jeremy Corbyn has consistently said no one earning less than £80,000 will pay more tax under Labour. But experts point out that millions of lower earners will pay more due to plans to scrap the marriage tax allowance and make changes to capital gains and dividends tax.

What are the proposed changes to dividend tax?

There is a tax-free allowance for the first £2,000 of dividend payments from shares. Above £2,000, the tax rate you pay depends on your income band, which is calculated by adding your dividends to your other taxable income. The basic rate is 7.5 per cent, the higher rate is 32.5 per cent, and the additional rate is 38.1 per cent. Labour says it will scrap the £2,000 allowance and tax dividends at the same rate as regular income.

... and for capital gains tax?

Capital gains tax is levied on the sale of an asset that has increased in value, such as a second home or share portfolio. It's the increase in value that's taxed, not the total sum and there is a £12,000 allowance each year. Basic-rate taxpayers pay 10 per cent on gains, and higher and additional-rate payers pay 20 per cent. Again, Labour plans to get rid of the allowance and bring the rates in line with income tax.

Who will this affect?

Anyone who has investments outside an Isa or pension. This is not just a problem for the rich because many people may not have got around to putting their money in an Isa, or may have had a windfall, such as inheritance. Pensioners surviving on small incomes could be vulnerable. Some may have downsized their home to release money to pay for their retirement, or taken advantage of new pension freedoms to cash in some of their tax-free lump sum that wouldn't be sitting in an Isa. A pensioner with an income of £14,500 - made up of a state pension worth £8,750, a private pension of £3,750, and a dividend income of £2,000 - would pay £400 a year more in tax under Labour.

What about small businesses?

Small business owners who earn less than £80,000 a year often pay themselves in dividends rather than salary because it is tax efficient. Because Labour plan to bring dividend tax rates in line with income tax rates, this advantage would be rendered redundant. The Federation of Small Businesses estimates that a company owner earning £60,000 would pay £3,000 to £4,000 a year more in dividend tax under Labour.

What would axeing marriage tax allowance mean?

The marriage tax allowance allows couples to transfer their personal income allowance between them, cutting the amount of tax they pay by up to £250 a year. To qualify, one partner must have a yearly income below the tax free allowance of £12,500, while the other must be a basic rate taxpayer earning £12,501 to £50,000 a year. The partner earning less than £12,500 can transfer up to £1,250 of their tax-free income to their spouse, which would otherwise be taxed at 20 per cent. Almost 1.8million people claimed it last year and stand to lose out, but the potential hit is greater for couples who have not used it so far because they may not be able to backdate their claims. Couples can claim up to five years' worth of the allowance, worth a maximum of £1,149 a couple.

I'd have to pay £6,000 more in tax

Gina Riley, the owner of Roleys Fudge in Chester, Cheshire

Gina Riley owns Roly's Fudge in Keswick in the Lake District, plus another small branch in Chester city centre.

Across the shops last year she earned just under £80,000, which she split between dividend payments and a personal salary.

She paid £12,374 in income tax in the past financial year, but, under Labour plans, she estimates that she would pay almost £6,000 more.

The 32-year-old said Labour's tax plans would reduce incentives for businesses.

'You don't feel like it's on your side.

'He [Corbyn] should be trying to encourage small businesses.

'In Chester there are more independent shops on the high street and anything that's going to hinder that is definitely not going to help.

She added: 'I've got two businesses, I've got a young family, it's very important to me.'

FRANCES COPPOLA: I've no axe to grind, but I'm staggered by Jeremy Corbyn's economic illiteracy

Economically illiterate. There's no other way to describe Jeremy Corbyn. During his BBC1 interview with Andrew Neil, the Labour leader revealed a failure to understand simple economics.

A total ignorance of how the tax system works, of the meaning of financial reserves and national debt, of government bonds, of pensions, of the impact that re-nationalisation would have – and above all of what life on a tight budget is like for millions of families.

As an economic commentator, I watched in increasing astonishment – and with great concern. The interview was immediately condemned as ignominious for Corbyn, by pundits on both Left and Right.

Economically illiterate. There's no other way to describe Jeremy Corbyn, writes Frances Coppola

But more than his dithering on Brexit, defence and anti-Semitism, it was Corbyn's breathtaking absence of understanding about money that frightened me.

I've no political axe to grind. I side with no party. But from an economist's standpoint, I am staggered by the breadth of his ignorance.

I particularly wanted to hear how he intended to pay for the 'Waspi' [Women Against State Pension Inequality] compensation handout that was announced, out of the blue, last weekend.

As I had suspected, there was no coherent plan to cover the estimated £58billion cost of this pledge – money to be added to the £83billion of other spending commitments in Labour's manifesto.

Time and again, Corbyn was left floundering at questions around basic financial matters.

Look at the way he talked about taking away the married couple's £250 tax break, as though that sum would be insignificant.

During his BBC1 interview with Andrew Neil (pictured), the Labour leader revealed a failure to understand simple economics, she writes

He appeared to shrug off the implications of losing that much money in one swingeing cut. Corbyn clearly does not realise that, according to the Office for National Statistics, £250 is more than the cost of four weeks' food shopping for an average family.

Only a man such as Corbyn –whose wealth has been calculated at £3million – could think such a sum insignificant.

His lack of understanding about life for those on a pension was equally shocking. Labour's plan for higher taxes on dividends would affect millions who worked all their lives to save just enough for retirement.

Corbyn appears to think that share dividends are just for 'fat cats', corporate businessmen who take advantage of the lower rate of tax. It's an infantile notion.

The reality is millions of pensioners rely on small but regular dividends from their pension portfolios. That's how they have been encouraged to save for retirement. To levy greatly increased taxes on those dividends would be monstrous.

Andrew Neil cited a case in which a pensioner scraping by on £14,000 a year would lose £400 at a stroke. Corbyn seemed barely to register what this meant. In his world, money doesn't have to be managed. It simply materialises when needed.

That was certainly the case when it came to Waspi compensation. Corbyn claims the Treasury must find up to £32,000 each for nearly four million women born between 1950 and 1960, to refund a pension shortfall. He calls this a 'moral debt', but there's nothing moral about it. On the contrary, his proposal is indefensible.

Labour is promising to pay this mind-boggling sum to the richest generation in history – while the worst of the burden will fall on people who are too young to vote in the General Election.

Labour leader Corbyn speaks during a BBC interview with Andrew Neil in London on Tuesday

As an economic commentator, I watched in increasing astonishment – and with great concern, Frances Coppola writes

It is clear that Labour has no serious economic plan.

Corbyn started blustering about 'government reserves', as though the nation has untapped billions in a savings account. It doesn't. What it has is a national debt currently standing at almost two trillion pounds – that's £1,750billion.

It happens that my 60th birthday is in January. When I began my career in banking, I expected to reach pensionable age at 60. Now, though, I must wait 'till I'm 66.

Labour's pollsters have identified women of my age as a demographic that might be tempted to swing to the Left, given a big enough bribe.

That is why the 'Waspi' policy appals me. Under Corbyn's proposals, I would receive £1,595 over five years. Yet women my age are part of the richest generation in history.

Of course, not every woman of my era can call herself well-off. As in every generation, some will be unlucky and hard-up, perhaps even living in poverty. But surely no other age group comprises so many wealthy individuals. As people live longer lives, the number with Alzheimer's and other forms of dementia rises. These people need all the care their loved ones can give.

If the Labour leader thinks he's got £58billion to spend, surely a living wage for carers would be a far more urgent priority.

Another alarming point was that Corbyn evidently had no idea of how reliant the country is on high earners to fund income tax. As it happens, 5 per cent of highest earners supply half the entire revenue.

Nor did he appear to have any conception of how nationalisation works. Taking the railways and utilities back into public ownership will have a huge impact on the life savings of ordinary people, as these are key assets for pension funds.

The cost of Corbyn's proposals barely seems to register with him. He talked of issuing government bonds to recompense shareholders, but couldn't grasp that these bonds represent more government debt.

That debt will have to be paid off, and meanwhile interest payments will push it even higher. Like an irresponsible spendthrift living on the 'never-never', Corbyn's only economic plan seems to be to put everything on the national credit card.

And that's a recipe for bankruptcy. There's clearly no economic rationale at all – in any part of his ruinous manifesto.

Frances Coppola is the author of The Case For People's Quantitative Easing, published by Polity Books