MPs are set to vote over whether to scrap the two-child benefit cap this evening, after an SNP amendment to the King's Speech was selected by the speaker.
A number of Labour rebels are expected to vote with the opposition party as a row rages on between them and the new government over whether to abandon the policy.
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While most on the Labour benches are against the measure - brought in by the Tories in 2017 to limit child benefit to two children per household - the leadership have said they can't afford to scrap it while public finances are in dire straits.
But MPs on the left of the party say it is essential to end the policy to help reduce increasing child poverty across the country.
Posting on X, former shadow chancellor John McDonnell confirmed he would vote for the SNP amendment tonight, adding: "I don't like voting for other party's amendments, but I am following [Sir] Keir Starmer's example, as he said 'put country before party'.
"So, I am putting lifting children out of poverty before party whipping."
The SNP's leader in Westminster Stephen Flynn also challenged other Labour MPs to "do the only right thing and vote with the conscience to end the two-child cap immediately".
The amendment has the backing of the Green Party, Plaid Cymru, the SDLP, the Alliance Party and a number of independent MPs, including former Labour leader Jeremy Corbyn.
But with a majority on 174, the government will still have the numbers on their side, even if some of its own MPs vote with their rivals.
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Tap hereSir Keir Starmer had previously said the cap should be scrapped, but he U-turned last year due to the state of finances Labour were set to inherit if they won the election.
There had been hints of a change in position amid the pressure on Monday when Education Secretary Bridget Phillipson told Sky News the government was "considering" removing the cap, although it was a "very expensive measure".
However, speaking to Sky News on Tuesday, Work and Pensions Secretary Liz Kendall said the government "would only make commitments when we show how we can fund them".
Instead, the government has set up a new taskforce to look into how to tackle child poverty across the country.
The Liberal Democrats have put down their own amendment today - also selected by the speaker - calling for the scrapping of the cap.
But the amendment also calls for a range of other policies that the party campaigned on in the general election, namely around social care, so Labour MPs would find it more difficult to vote for it.
Votes are expected to take place tonight at around 7pm.
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