Liberal Democrat health spokesman Simon Hughes is to press the two NHS London regions to stop the London Lighthouse centre for people with HIV and AIDS from being sold in September.
He told a House of Commons rally that the regional offices should do more to persuade London trusts and health authorities to prevent the sale since its services were used by people from across the capital.
Lighthouse managers decided in March to put the building up for sale after the withdrawal of£1.7m in NHS funding made continuing its residential care services unviable.
The charity is exploring with a local housing trust the idea of selling part of the building for social housing so that enough space can be kept to meet non-residential needs of service users.
Lighthouse chief executive Susie Parsons told the rally that the charity needed to raise up to£700,000 to avoid the sale.