The reports, being sent to all Labour MPs, conclude that bidding for contracts is causing many charities to abandon their campaigning activities, lay off staff and reduce working conditions for those that remain.
The documents also cast doubt on the sector's ability to take over large numbers of public services, and describe the evidence that it is a better service provider than the public sector as "thin".
Mike Short, Unison's national officer for the voluntary and community sector, said he believed the Government was sincere in its desire to embrace the sector's independence, innovation and capacity for advocacy. But he said the direction of policy was often misguided.
"A lot of proposals still damage the sector and its ability to provide services," he said. "The gradual but definite shift of funding to state funding, especially by contracts, inevitably changes the way voluntary sector organisations behave.
"Lots of employers are getting rid of staff and public advocacy officers so that they can concentrate on winning contracts."
The first report, False economy? The costs of contracting and workforce insecurity in the voluntary sector, is based on focus group discussions with junior and senior voluntary sector staff in a variety of cause areas. The second, Third sector provision of local government and health services, is an academic study by Cardiff School of Social Sciences researcher Steve Davies.
Short said the second report put "the meat on the bones" of the argument put forward by Unison general secretary Dave Prentis in Third Sector earlier this year. Prentis said: "Competition for public services and the marketisation of the third sector are damaging the essence of both the services in question and the voluntary sector that is increasingly providing them."