Social care staff at the housing association Family Mosaic are facing pay cuts and longer working hours in move branded "appalling" by Unison.
The London and Essex housing provider is requiring about 40% of its staff to sign new contracts under which working hours would rise from 35 to 37.5 a week. About 20% of employees would face pay cuts.
Family Mosaic blamed the changes on the government's austerity measures, which it says have resulted in "significant cuts" to its grant income for care and support services.
But Nick Bradley, Essex Unison's regional organiser, accused Family Mosaic of "acting like bullies" because staff who refuse to sign the new contracts will lose their jobs.
Colin Inniss, regional organiser at London Unison, added that the changes would "inevitably affect staff morale at Family Mosaic and lead to a diminishing quality of service delivery".
The trade union intends to fight the changes and is considering a strike and legal action.
Family Mosaic chief executive Brendan Sarsfield said: "We want to continue to provide high-quality services and support but had recognised that a terms-and-conditions review was essential if we were to protect jobs and services long term. Even after these changes, we will still have a very competitive package to offer existing and new staff."
A spokeswoman for the housing association said the increased hours were part of a move to standardise staff hours and some employees would work fewer hours as a result. She also said that the salary reductions are being phased to lessen their impact on employees and that no-one would have their pay cut by more than £1,000 before April 2013.
Unison, however, said the pay cuts could amount to as much as £11,000 a year for some employees.
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