Messengers at the Home Office HQ in central London are battling privatisation.
They face transfer to Ecovert FM. Ecovert has told workers that they will be transferred under TUPE regulations.
The firm has said that it will honour present terms and conditions, but has admitted that it is free to alter them.
The workers are already some of the lowest paid in the Home Office.
The PCS union is organising a campaign against the plans. A strike in 1997 beat off a previous privatisation attempt.