Sinn Fein will try to delay welfare reform measures being considered by the Northern Ireland Assembly next week.
The Welfare Bill is flawed and targeted at the most vulnerable and disadvantaged in society, MLA Alex Maskey said. He linked it to "austerity policies" of the Conservative-led coalition at Westminster.
Key features include a universal credit to cover a range of benefits, a personal independence payment reassessed every three years to replace Disability Living Allowance and housing benefit reforms.
Mr Maskey said: "It is our view that the Assembly has a duty to stand up for low-income families and those on benefit who are being directly targeted by many of these measures.
"The fact that these measures are being brought forward by the Tory Government and being imposed here is a direct consequence of the Assembly not having the necessary fiscal powers."
Sinn Fein will be bringing forward a proposal to the Assembly on Tuesday to defer the Welfare Reform Bill until significant amendments are made.
Mr Maskey added: "We will press for fundamental changes to this Bill to ensure the maximum protections for those on benefits and in low-paid employment." The impact of Sinn Fein's intervention will depend on whether other parties support them.
DUP Finance Minister Sammy Wilson MLA said it was a bombshell from Sinn Fein.
"Only last week Sinn Fein Ministers agreed that the Welfare Reform Bill could be brought to the Assembly. Indeed, they recognised that the timetable was tight. The consequences for delay of the Bill will be felt by welfare claimants, civil servants and public spending across all government departments.
"Sinn Fein took so long to agree this Bill at Executive level there is no slippage time at all available if the Bill is to clear the Assembly and be in place for April 2013. If it does not receive Royal Assent by April, then immediately 250,000 people who benefit each year from the Social Fund will find themselves with no means of support."