Up to two thirds of Brent families could be left struggling if plans to cut tax credits go ahead, according to charity Barnado's.
The organisation is urging politicians to keep the "lifeline" benefit and not cut tax credits.
Ahead of the Budget speech on Wednesday (July 7), rumours have circulated that the Government will reduce the benefit, with some families losing it completely.
Barnado's has calculated that 24,800 families in Brent - 65.1% - currently use tax credits to top up low incomes, helping them buy essentials such as food and clothing for the family.
If the rumoured cut to tax credits goes ahead, Barnado's have estimated that around 49,500 children in the borough stand to be affected.
More than half of Ealing families could be left struggling if the axe goes ahead.
The charity said 23,600 families in Ealing (55.9%) currently use tax credits to top up low incomes.
More than half (54.5%) of families with children in Greater London receive tax credits.
Brent has the fifth highest proportion of families receiving tax credits in the capital, behind only Tower Hamlets, Newham, Hackney and Barking & Dagenham.
Barnardo’s chief executive Javed Khan said: "Without this income, many parents in London could not afford their weekly food shopping let alone school uniforms and books.
"With low wages and high living costs stretching budgets, tax credits are an everyday lifeline for British families.
"Children who grow up poor are more likely to be ill, do worse at school and be jobless in future. If as a society we fail to invest in children now, we will all bear the costs in the future.
"Families would be better off if the government focused on tackling low wages and high childcare costs, instead of cutting struggling families income."
Tax credits, which include child tax credits and working tax credits, were introduced in 1998 as a response to rising child poverty, caused by low wages and high living costs.
Since the introduction of tax credits, the number of children living in poverty in the UK has fallen from 35% to 19%.