Claimants in the London boroughs of Haringey, Enfield, Croydon and Bromley are the first to face a cap on their benefits.
Employment minister Mark Hoban said it was "only fair that those who are on benefit face the same choices" as those in work.
But shadow minister for disabled people Anne McGuire highlighted the "major issue of imposing that level of benefit cap in London, where the biggest factor in benefits is actually the high rents."
Political correspondent Libby Weiner reports:
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Police are trying to find the driver of the car following the incident in Kingfisher Close, Brownhills.
An unnamed man died at the scene, while a woman was pronounced dead after being taken to St George's Hospital, Tooting, south-west London.
Eastern areas will see a few showers coming in on the northeasterly breeze