Labour has attacked "shameful" cuts to tax credits as the Government faces a possible Commons rebellion over a flagship part of the Chancellor's summer Budget.
Around five Tory MPs are reportedly planning to vote against the plans, which face concerted opposition from Labour, the SNP and other minor parties.
The Government has a slender working majority of just 16 leaving it vulnerable to even small rebellions of Conservative MPs.
Seema Malhotra, the new shadow chief secretary to the Treasury, condemned the plans which she said would leave millions of households out of pocket.
Treasury Minister Damian Hinds defended the plans as crucial to the Government's wider ambition to tackle low pay at the start of the 90 minute debate, with a vote due at 5.30pm.
Ms Malhotra said: "10 million people, a sixth of the population, will be affected by these changes and every member of this House represents at least some of those who will be hit - around half of working families in our constituencies.
"However it is heartening to read in media reports today at least five MPs of the Government backbenches are planning to vote against these changes.
"It is certainly not too late for the Government to change its mind."
Ms Malhotra told the Commons the changes were "highly controversial", adding: "These cuts to tax credits hit working families in every constituency and were to be sneaked through the back door.
"This is a political decision made by the Chancellor that is set to see over three million families lose an average of £1,000 a year. It is ideologically driven, it is cynical and it will directly increase levels of poverty in Britain."
Ms Malhotra added: "It is part of an on-going attack on the incomes of some of the most hard working families in our constituencies - those very strivers the Chancellor purported to support.
"It is shameful what we are seeing is a cut in the incomes of the poorest in our constituencies."
The changes to tax credits being voted on by MPs are expected to save £4.4 billion in 2016-17.
In the Budget Mr Osborne announced plans to cut from £6,420 to £3,850 the earnings level above which tax credits are withdrawn from April 2016, as well as speeding up the rate at which the benefit is lost as pay rises.
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