The SNP has stepped up demands for the so called rape clause to be ended "for good" after a United Nations Human Rights report called for its repeal.
The report, by the Committee on the Elimination of Discrimination against Women (CEDAW), made a number of recommendations including that the two-child tax credit limit be repealed.
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The UK Government has already halted the roll-out of the policy after a fierce backlash - but a senior Nationalist MP said ministers must go further.
Glasgow central MP Alison Thewliss said: “This most recent condemnation is a damning confirmation of what is a truly cruel and pernicious policy by this heartless UK Tory Government.
“Having ceased roll-out of the policy to third and subsequent children born before April 2017, the DWP Secretary of State Amber Rudd must now recognise that the two child policy is unfair for everyone who is affected by it.
“No one can plan for the whole course of their family life, and social security should be a safety net for all of us when we need it.
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The measure limits child tax credit to the first two children. A number of exceptions were set out, including for a child born as a result of "non-consensual conception".
Work and Pensions Secretary Amber Rudd announced a rollback in January, but faced claims that she was creating “two classes of family” by scrapping it for some claimants but not others.
Ms Thewliss added: “I will be writing to the UK Government to ask for immediate action on CEDAW’s findings. Amber Rudd must do the right thing and end the two child limit for good.”
A DWP spokesperson said: “The two-child policy ensures fairness between claimants and taxpayers who support themselves solely through work. Appropriate exceptions and safeguards are in place.
“On 11 January 2019, the Secretary of State announced measures that will provide increased support for Universal Credit claimants, including the removal of the extension of the two-child limit on Universal Credit for children born before 6 April 2017. All children born before that date will continue to be supported by Universal Credit.”