On behalf of 102,000 members in Prospect, General Secretary Paul Noon told the meeting that while not opposed to the private sector, the union did not believe it had a role to play in public services.
"We are not standing in the way of change, but what we oppose is change that harms public services, denudes the government of specialist skills and increases costs to taxpayers."
Noon said that Labour had reneged on pledges given in 1997 to reaffirm its commitment to the public services, halt privatisation and put a moratorium on market testing and contracting out.
Instead, he said, it had introduced a public private partnership for air traffic control, leaving controllers facing a daily battle to stop commercial pressures interfering with operational safety, privatised naval bases and closed essential scientific institutes.
"It has even investigated organisations for privatisation not even thought about under a Conservative government, such as the Forensic Science Service," he said, warning that the result was the loss of the in-house expertise the government needs to act as an intelligent customer.