The tax changes have led to strong political debate
The abolition of the 10p income tax rate has prompted strong debate among politicians.
Labour could be facing a backbench rebellion over the move, and the Conservative leader David Cameron has vowed to fight to make the government think again about the change.
But abolishing the rate was part of a series of measures alongside tax credit and pension changes which had helped the low-paid, according to Chancellor Alistair Darling.
It has meant people who would have paid income tax at the lowest, introductory 10% rate will now have to pay the 20% rate, but child benefits, state pensions and tax credits have all gone up.
Prime Minister Gordon Brown urged people to look at the move in the "wider context" of tackling poverty - but as with any tax changes, there have been winners and losers.
A series of tax changes came in at the start of the financial year, on 6 April. They had been announced in Gordon Brown's final Budget as chancellor in 2007 and in the later pre-Budget report.
With the effect now hitting home, the strength of feeling among MPs is being heard during debate on the Finance Bill.
The changes saw the 10% starter rate of income tax abolished, except on savings income for people who are low paid.
The basic rate of tax fell from 22% to 20% for everyone.
As well as a rise in everyone's personal allowance, age-related allowances for those aged 65 or older have seen a big increase, from £7,550 to £9,030, and to £9,180 for those 75 and over.
The main National Insurance rate of 11% is being levied on more of people's incomes, up to £40,040 a year.
Essentially, working families are the ones who have done well out of the changes.
The Institute for Fiscal Studies (IFS) estimates that, taking all the changes in income tax, national insurance and tax credits together, 55% of households gain at least £1 a week.
WINNERS AND LOSERS FROM INCOME TAX, NATIONAL INSURANCE AND TAX CREDIT REFORMS
24,000 households losing more than £10 a week
3.58 million households losing £1 to £9.99 a week
7.96 million households within £1 better or worse off a week
12.12 million households gaining £1 to £9.99 a week
1.78 million households gaining more than £10 a week
Large gains of greater than £10 a week are most common for families with children.
A household with children where more than one resident is on benefits would lose an average of 54p a week owing to income tax and national insurance changes, but gain £4.63 from reforms in child tax credit and working tax credit.
The vast majority of pensioner households are either better off or unaffected by the changes as a whole.
The IFS figures show that the richest half of the population gain from changes to income tax and national insurance. The biggest gainers are the richest 10th who gain £6.84 a week.
PricewaterhouseCoopers say that those with incomes over £18,000 are winners when looking at the income tax changes alone.
Losers from the changes will be those who do not receive either the working tax credit or the child tax credit but who have incomes between £5,435 and £18,500 per year.
So this is mainly childless single adults aged under 25 and childless adults working less than 30 hours per week.
The latest row has been centred around 5.3 million people losing from the changes.
They are made up of 2.2 million single people without children; 1.2 million working couples with no children; 400,000 childless couples with one earner on around £17,000 to £18,500; 700,000 couples, both earning, with children; 300,000 women aged 60 to 64 who have retired and are on low incomes, and 500,000 people under pension age and out of work who have taxable income.
Some backbenchers are unhappy that, looking at just income tax and National Insurance numbers according to the IFS, the poorest half of the population lose between 43p and 85p a week - although they gain more in working tax credit and child tax credit.
The IFS says that, on average, 14% of all households lose more than £1 a week.
The number of people losing out could be greater owing to the poor take-up of some benefits, the IFS says.
Groups such as the Low Incomes Tax Reform Group say the complexity of the rules causes confusion.