The FSS deals more than 120,000 cases a year
Union officials have attacked an announcement by the government-owned Forensic Science Service that it will close three of its seven laboratories.
The FSS, which analyses crime scene evidence for England and Wales police, said it would shut sites in Lancashire, south Wales and West Midlands.
The Prospect union said it was "deeply disappointed" by the loss of 700 jobs.
But FSS Chief Executive Officer Dr Simon Bennett said the move was vital to "maintain a sustainable business".
Under the plans announced by the FSS, laboratories in Chorley, Lancashire, Chepstow, Monmouthshire, and Birmingham, West Midlands, would close.
In a statement, the FSS said each of its four remaining laboratories would concentrate on specific crime "streams" such as DNA, sexual offences and major incidents, such as terrorism.
Previously, Prospect had warned that the cuts would put extra pressure on the criminal justice system and leave large areas of England and Wales without public sector forensic cover.
Helen Kenny, the secretary of Prospect's FSS branch, said: "Our members are deeply disappointed by this decision and that the union's alternative case, to retain seven sites but with a trimmed headcount, has been dismissed.
"The cuts are driven by claims that the workload has diminished, which we don't accept - no-one has seen the crime rate go down."
Ms Kenny said the union would consider what action to take in light of the announcement.
She added that although the 700 job losses meant a reduction in the original estimate of 800, some 50 members of staff had left since the start of the consultation process in June.
But Dr Bennett said the service needed to take account of the fact that the market was changing.
He added: "As the largest provider, we aim to continue to fund valuable research and development, to bring ground-breaking techniques to crime fighting, but at the same time, the new, leaner structure will focus on what matters: providing the best and most cost-effective service possible to support the police."
The service deals more than 120,000 cases a year.
In 2005 it changed status from a government agency to become a government-owned company in 2005, meaning that it had to bid for work against other forensic service providers.