Blank passports and visas have been stolen from a van travelling from Manchester to London.
The Foreign Office has confirmed 24 parcels were taken and that could mean thousands of individual documents.
A spokeswoman described the incident as a serious breach of security.
She the blank documents were destined for British embassies and were being transported from a factory in Oldham to RAF Northolt in northwest London.
The Foreign Office spokeswoman said: "The van was hijacked en route from the production site."