Chief Treasury Secretary Danny Alexander is to accuse trade unionists of "spoiling for a fight" over the coalition's efforts to tackle the deficit.
Mr Alexander is to warn that some elements are trying to "misrepresent" the Government's aims as he appeals for public sector workers to co-operate to push through sharp cuts in public spending.
The intervention comes on the first full day of what is set to be a stormy Liberal Democrat conference in Liverpool, with activists nervous about the compromises made to share power with the Tories.
More than 2,000 TUC members are due to protest near the Arena and Convention Centre over the coalition's plans for deep spending curbs.
But Mr Alexander will urge workers not to seek a confrontation, insisting ministers do not want to "take on" staff.
"I would like to say one thing to nurses, teachers, police officers and civil servants. Thank you," Mr Alexander is to say.
"Your ideas, your effort and your commitment are essential in helping people get the best from the services you provide.
"I know that the next few years will be tough, very tough for some.
"But I also believe that the changes we make - empowering you, trusting you, listening to you - will make the public services a more rewarding place to work.
"I know there are a minority in the trade unions who will deliberately misrepresent what this Government stands for because they are spoiling for a fight. Please don't allow their political motivations to push you into doing the wrong thing for the country. We do not want to take you on. We want to take you with us."