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:
More top news
Very windy with gales possible
Strong performance from world number 100 meant Great Britain very nearly dominated day one of the competition.
As a promotion Black Friday is flawed. While it whips up demand and redistributes it the event doesn't obviously generate additional sales.