Labour has accused the government of "pulling the rug" from underneath hundreds of thousands of poorer families as it attempts to block changes to Universal Credit.
Shadow education secretary Angela Rayner told MPs that more than 1 million children would be entitled to free school meals without the proposed changes by ministers.
But work and pensions secretary Esther McVey accused Labour of "scaremongering and misinformation", saying an extra 50,000 children would benefit under the plans.
The government wants to introduce a means test for those on Universal Credit.
It would mean claimants with net earnings up to £7,400 would be entitled to free school meals.
Families are currently able to claim free school meals if they are on Universal Credit, as part of a package of measures to help the transition to the new benefits system.
'Not all children in poverty get free school meals'
"Many people, including MPs, wrongly believe that all children in poverty already get free school meals," Ms Rayner told the Commons.
"That is not currently the case, but under the transitional protections under Universal Credit that there would be, those million children would be entitled to that benefit.
"It's through this secondary legislation that they're pulling the rug from underneath those families."
She added: "The current system would help over a million more children than under the plans we would vote on today."
MPs heard it would create a cliff edge in Universal Credit: families would be better off earning less in order to qualify for free school meals.
The issue in Northern Ireland was also raised, where a more generous threshold of £14,000 is due to be upheld.
"Under these plans, Universal Credit will mean that work does not pay for hundreds of thousands of families," said Ms Rayner.
The Department for Education has estimated that by 2022 around 50,000 more children will benefit from a free school meal, compared with under the previous benefits system.
"While I understand that it is the nature of the opposition to oppose, scaremongering and misinformation from that side of the House have surely reached a new low," said Ms McVey.
"Unsubstantiated and exaggerated claims about widespread problems caused by Universal Credit amount to nothing less than scaremongering.
"They cause claimants alarm and, in the worst cases, stop them getting the money they are entitled to.
"And yet where we find ourselves today, once again debating Universal Credit with the same false alarms from the shadow Cabinet."
Former Tory leader Iain Duncan Smith, who led the Universal Credit reforms, added: "Transitional protection was designed to protect those moving from tax credits into Universal Credit so they did not, if it happened to be the case, lose any money in the transition.
"It was not about increasing, to the degree that she's talking about, the number of those who'd receive free school meals.
"Under Universal Credit, more will receive free school meals than there would have been under their plans."
The change is one of four government plans that Labour has called for a vote on in the Commons. They are proposals to end childcare vouchers and instead use the tax system, as well as changes to free childcare for two-year-olds and further changes to universal credit.
SNP social justice spokesman Neil Gray said he was "deeply concerned" about the plans to close the childcare voucher scheme to new entrants from April.
He added: "It's completely unfair to expect people on low incomes to not only have to cope with the fact that their benefit is likely frozen and not meeting their costs of living, but also that the government continue to add additional layers of punitive bureaucracy designed to trip them up ... it is underhand and it is appalling."
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