Key safety functions under threat include: lorry and bus testing; training services; operator licensing administration; prosecution and legal services; traffic, vehicle and MOT enforcement administration; and routine enforcement.

The warning comes after management at the agency revealed that a review of outsourcing options had identified a public private partnership as the preferred way forward. But Prospect’s 580 members in the agency remain unconvinced over the need a PPP and are lobbying for a full and open parliamentary debate examining the case for keeping VOSA in the public sector.

The Transport Select Committee has echoed the fears of Prospect members in its annual report for 2006, which stated: "We have serious reservations about proposals to outsource any testing and enforcement responsibilities. Safety must be the top priority."

The move is being driven by a call for efficiency savings. Yet the union estimates that one major incident avoided through increased enforcement would save £1m of delay costs, while 50 per cent of critical incidents – such as side swipes, insecure loads and tiredness – could be addressed through VOSA inspections.

VOSA section secretary Kevin Warden said: "The UK currently has a safety record to be proud of but we have real fears that standards will slip if this privatisation goes ahead. You only have to look at the countries of origin of most of the defective HGVs entering the UK to see that those are countries where VOSA type activities have already been outsourced.

"Our members are determined to fight to ensure that the largest and heaviest vehicles on our roads, and the roadside enforcement of those vehicles, is not undertaken by a company looking to create profit rather than VOSA’s objective of a safer environment for us all."

Prospect negotiator Jeremy Windust said: "The current state of the railway system is hardly a recommendation that transport systems are better off in private hands. Operators and government need to ask themselves how much they value safety in the road transport industry and how far they are prepared to go to achieve this."

The union wants input into the public sector comparator against which any PPP proposal will be measured. A ministerial decision is expected by early summer.