The Prime Minister's vision of a "big society" should include the UK's seven million trade union members, an official will urge.
Paul Noon, general secretary of Prospect, will say the Government needs to listen to "the voice of the professional", democratically expressed at the workplace through trade unions.
In his keynote address to the union's biennial national conference in Liverpool, Mr Noon will also call on employers to heed the views of engineers, scientists, managers and other professionals on how their organisations should be run more effectively.
"Trade unions have been doing the big society for 200 years," he will say.
"If the Prime Minister is serious when he talks about encouraging volunteering and involvement in social action, then he must work with trade unions, not against them.
"We have already made it clear we seek engagement not confrontation. The Government has its programme but we have a legitimate right to speak and act on behalf of members and in defence of the services they provide."
Mr Noon will say that ministers had a choice on public services, continuing: "They can either listen to the views of their staff constructively expressed through their unions and be flexible.
"Or, if they are foolish, they will seek to impose top-down, unacceptable arrangements which will deserve and attract our outright opposition."
The 500 delegates will hear that the public sector cuts threatened by the new coalition Government will lead not only to the loss of vital services but also damage business for contractors in the private sector.
