Julie Corbett-Bird and Jenny Hinds of Blackfriars Settlement told Southwark's cabinet on Tuesday night that the planned cuts to community services are more severe and more wide-ranging than feared.
Blackfriars Settlement is a 124-year-old local charity that provides a range of services for young people, older people and people with mental health problems.
"We know that we have to bear some cuts," director Julie Corbett-Bird told Southwark's 9-member Labour cabinet.
"We have budgeted for cuts. It is the extent and breadth of the cuts that potentially creates a problem for our organisation.
"The fact that we are multi-purpose – that we receive grants from a range of different departments – clearly has a cumulative impact for the organisation which means that it is not just the staff in particular projects that are affected.
"I have been asked by my trustees to send a note out to all staff – including myself – advising them of the potential risk of reduction of hours or loss of post. It could have a very dramatic effect on us."
She highlighted the organisation's long-term commitment to involving volunteers in its activities.
"In reference to the Government's Big Society, we have 100 volunteers working with us," she said. "We have a free legal service that's delivered by a law firm and students from the Inns of Court School of Law.
"We have a lot of volunteers; we could have more if only I had someone who could support them. It is the nature of the history of our organisation that we use volunteers effectively.
"We have not been blind to what has been going on and what is happening here. We had an external review carried out last year to identify potential efficiencies.
"We are developing our site at Rushworth Street in partnership with Notting Hill Home Ownership to provide up-to-date, modern, accessible community space and improve anchor provision in the area, a role that we are developing in partnership with Bankside Open Spaces Trust and Bankside Residents' Forum."
She called on the council – and its NHS partners – to engage in more dialogue with community organisations as it prepares to make cuts at the same time as the health service is reorganised.
Ms Corbett-Bird also voiced concerns that the council's proposed reorganisation of youth services was unlikely to be completed in time for the start of the new financial year in April.
Council leader Peter John said that he had encouraged officers to communicate well with organisations that receive funding from the council: "One of our budget principles is that we will inform people as soon as possible and I hope that that has happened more often than it hasn't."
Ms Corbett-Bird concluded by issuing a warning to councillors about the possible consequences of a lack of clarity about the future funding of services: "The fact that you could lose a lot of expertise in that intervening period is something that I think the council really does have to take on board," she said.