Prospect, the union for professionals, and the Public and Commercial Services union said a decision on the future of the agency was expected before the summer recess on July 17.
They have pledged to fight the proposals as damaging to the war on crime, costly for the police, a threat to science standards and demoralising for staff.
FSS, now a Home Office trading fund, analyses more than 135,000 samples of forensic evidence for police forces in England and Wales each year, as well as 555,000 samples of DNA. It has 2,700 staff, including 1,600 scientists, based at seven laboratories in Birmingham (two), London, Chepstow, Chorley, Huntingdon and Wetherby.
The agency last week announced 229 redundancies by October 31, which Prospect negotiator Mike Sparham said was "clearing the decks for privatisation." He added: "FSS more than covers its costs and this year the National Audit Office said its services are rated highly by an overwhelming majority of users.
"There is no demand for this privatisation, which is profoundly opposed by staff and would disrupt vital forensic work for the police all over the country.
"Far from being tough on crime, this proposal would be tough on the solution to crime. We call on everyone working with Britain’s system of criminal justice to ask the Home Secretary to block this crazy idea."
No other country in the world operates a privatised forensic science service, the unions point out. The review fails to identify a potential partner for the PPP, which is not surprising given that FSS is far and away the dominant player in the UK forensic science market.
The review team’s main justification for the PPP option is that it would enable the £20m-£30m needed to modernise facilities at the seven FSS laboratories to be raised from the money markets rather than government.
"That is almost exactly the size of David Beckham’s transfer fee to Real Madrid," said Sparham. "What a pity the review team or the Treasury, whoever is behind this scheme, does not think forensic science is worth an equivalent investment."
