Official figures show that 3,300 households in Brent are set to lose benefits compared with 1,500 in Birmingham.
Birmingham has a population of more than one million, around four times as many people as in the north-west London borough.
Minister for children and families Sarah Teather, who is Liberal Democrat MP for Brent Central, has warned that the cap could force people out of their homes and was absent during the key votes on the welfare reforms.
The figures from the Department for Work and Pensions also appear to undermine claims that the cap could lead to an exodus from central London to outer boroughs.
Four out of six areas that will be worse hit than Birmingham are outer London boroughs. They are Brent, Ealing with 2,200 households affected, Enfield 2,200 and Newham 2,100. The other two boroughs are Westminster, 2,800, and Tower Hamlets, 1,700.
The cap is backed by a majority of voters who support the Government's view that people should not get more in benefits than workers earning £35,000 a year before tax.
A DWP source said: "The cap is there to restore fairness to the system that has spiralled out of control.
"These figures show that it is simply not the case that families on benefits will be forced out of parts of central London as some people claim. It does show, that benefit claimants, like those who work, will face decisions about where they want to live based on what they can afford."
But London will bear the brunt of the cap, with 36,000 out of 67,000 affected households across the country in the capital.
Town halls in London have called for the cap to be "regionalised" to lessen its impact on the capital.
Ministers have argued that some people may have to move but expect them to be able to find other accommodation in the same or a neighbouring borough.
Despite the higher housing costs in central London, 1,400 homes in Brent compared with 1,000 in Westminster will lose more than £100 a week.
An impact assessment of the reforms showed 17 boroughs will see more than 1,000 households affected including Barnet, Camden, Westminster, Croydon, Hackney, Hammersmith & Fulham, Haringey, Harrow, Islington, Kensington and Chelsea, Redbridge and Wandsworth.
Peers led by bishops inflicted several defeats on the Government on the welfare reforms in the Lords but ministers have vowed to overturn their amendments.