Former Scottish secretary Alistair Carmichael has uncovered figures showing nearly 2,000 families in Orkney and Shetland would lose a total of £1.4million under the proposed tax credit changes.
Figures from the House of Commons library also reveal 3,300 children in the Northern Isles would be affected, the Lib Dem MP said.
Mr Carmichael described working tax credits as “lifeline benefits” to those on low or middle incomes “striving to provide for their families”.
“They are the very people that just a few months ago David Cameron was claiming to stick up for,” he added.
“The prime minister points to increases in the minimum wage and the amount that you can earn before paying tax as the means by which these cuts in tax credits will be restored.
“The problem is that these changes will bring in money in the future but these cuts are in the here and now.”
He said he had never “shirked” from the need to bring public spending under control.
But he added: “In government, Liberal Democrats constantly blocked this sort of change because we felt that it hit the poorest hardest.
“Now that the Conservatives are a majority government they can do whatever they want.”
The Orkney and Shetland MP also branded the cuts unnecessary on the grounds that as wage increases kick in, the amount paid out in tax credits will automatically fall.
He went on: “I have heard that there is now plenty of disquiet on the Conservative backbenches about the political cost of these reforms.
“I hope that the chancellor will see some sense and row back on these plans.”