In its evidence to the Greater London Authority review of London weighting, it warns the capital could face a widescale recruitment and retention crisis.
Unison has pointed out that someone earning£30,000 a year could only afford a house in one London borough. The key worker scheme, though welcome, only helps a relatively small number.
The union is calling for a uniform rate of London weighting for all public sector staff.
The next meeting of the GLA advisory London weighting panel is on Wednesday.
-- Unison has won the backing of European trade unions representing more than 40 million people in its fight for fair wages.
Unions across the continent are backing Unison's campaign to protect British public service workers during the government's privatisation drive. The European Parliament has already passed a clause requiring companies to preserve public sector working conditions, but it still has to be passed by the council of ministers.