Former leadership contender Diane Abbott is set to be among the MPs planning to defy the Labour whips when the vote is called this afternoon.
Ms Abbott (Hackney North and Stoke Newington) said she would not support the cap because it was the wrong thing to do.
Everyone wants the welfare bill to come down because spending on welfare is the price of failure.
However, we think this cap will just encourage arbitrary cuts rather than long term policies because that will bring down welfare spending.
We know, because of research by the Chartered Institute of Housing, when they did four pilots of the cap, in Haringey only 10% of households were known to have found jobs and 50% had to get more money from the council.
It's also part of the narrative to demonise benefit claimants. I don't think we should allow George Osborne to play politics with this issue, because it is people's lives.
Asked if Labour backing the plan would open the Opposition to the same charge, Ms Abbott said: "I think there is a danger it would be seen as playing politics."
MPs have voted in favour of the government's plans to cap welfare spending - her is how it will work.
The mother of a disabled child described how she struggles to make ends meet after having her housing benefit cut by 14%.