London will lose nearly £420m for social services under a government funding proposal, according to the organisation London Councils.
Funding for helping the disabled could be cut
It said the government was considering scrapping a protection called damping, which keeps funding above a set level.
It said the cut would hit provision for young adults in areas such as mental health and helping the disabled.
A government spokesperson said it was consulting on phasing out damping and a decision will be made later this year.
London Councils said the fall in funding would hit 90% of London's 32 boroughs and would mean the city would receive funding for just 68% of its social care expenditure.
It claims a collection of authorities in the North West, West Midlands and Yorkshire and Humber that are set to benefit from the scrapping of damping, were putting pressure on government to remove it.
London Councils chairman, Councillor Merrick Cockell, said: 'If the government bows down to pressure and applies the raw funding formula without funding protection, it could have serious long-term implications for the future provision of social services for vulnerable adults throughout the capital."
Local Government Minister John Healey said: "Councils in London received an average 3.4% grant increase in this financial year, well above the 2.7% floor, and local councils across the country have received a 39% real terms increase since 1997.
"We have moved to a fairer system of funding which takes better account of local need, however, to ensure that councils in London received more grant than last year we have phased in these changes through the damping scheme.
"We have consulted councils on whether the damping scheme should be phased out as part of the next three year settlement and we welcome their views.
"The settlement will be announced by the end of this year and it is not possible for anyone to speculate on figures for individual authorities at this stage."