Mark Serwotka, general secretary of the Public and Commercial Services union, which represents many job centre employees, rejected a denial from the work and pensions secretary, David Blunkett, who said there were no plans for "wholesale privatisation of job centres".
Speaking at the TUC conference in Brighton, Mr Serwotka was unconvinced by Mr Blunkett's response to revelations by the union that up to 60,000 posts could move to the private sector under Department of Work and Pensions plans to outsource some job centre functions.
He said: "It's clear to me that the 'denial' was much less than unequivocal. What's shocking is that this plan to part privatise job centre work was actually Tory party policy, as put in their general election manifesto just three months ago. Now it appears to be government policy.
"Now what David Blunkett appears to be saying is that job centres won't be privatised, but there are thousands of workers behind that working on processing the claims and they could be in the firing line."
Mr Serwotka said strikes were possible if moves were made to privatise job centre work.
"Let's be clear," he said, "part privatising the job centre network would be one of the biggest attacks on the welfare state in living memory. If it happens, we will consult for a far reaching campaign and potentially substantial strike action."
Referring to the chancellor's address to the conference tomorrow, Mr Serwotka said: "My union is already threatened with 80,000 job losses, so I'm not taking any lectures from Gordon Brown."
The chancellor was earlier told by other union leaders that they would not necessarily rubber-stamp his succession as Labour party leader.
Yesterday, Mr Blunkett responded to a leaked letter that appeared to show his department was planning "feasibility studies ... to ascertain whether there might be scope for contesting or outsourcing benefit processing centres, fraud investigation services and both jobseeker and employer contact centres".
He said that putting some functions out to tender had been policy "from the inception of the New Deal programme".
Meanwhile, Mr Serwotka said that the welfare of workers would have to come first in plans to create a "super union" by merging the GMB, Amicus and TGWU.
"The formation of a super union clearly has wide repercussions for both the government and the trade union movement. For me, the test is whether it's going to improve representation for people on the shop floor and in the workplace, whether it will give them a united, stronger, response.
"But it's a decision for those trade unions to make for themselves. And it's not about influence within the TUC, or card vote clout, but what difference it will make for the workers of Gate Gourmet, or Rolls Royce."
