Former local government secretary Stephen Byers told delegates at a conference organised by the Social Market Foundation there should be limits on the private sector's involvement in public services.
He said the government's failure to define the extent to which PPPs can be used to improve services risks damaging other initiatives such as the introduction of foundation hospitals.
A spokeswoman for Unison said Mr Byers' comments, made on the anniversary of his resignation from the then DTLR, are significant because he is seen as a staunch Blairite.
Mr Byers' sentiments were echoed by Audit Commission chair James Strachan in an interview with The Financial Times, in which he said the government's determination to use firms to prop up struggling public services means 'billions of pounds of public assets and funds [is going] into highly nascent structures'.
A spokesman for the 4Ps, which advises councils in England and Wales, said: 'Our function is to enable government policy, rather than decide what it should be, but partnerships do tend to deliver value for money in the majority of cases.'