Thieves who got away with 3,000 blank passports and visas worth around £2.5 million targeted the van as it stopped at a newsagent's, police have said.
The two unarmed thieves assaulted a security officer before driving off in the van in Oldham on 28 July.
Greater Manchester Police have launched an investigation and are appealing for eyewitnesses to come forward.
The Foreign Office has admitted a serious breach of security over the loss of the documents.
The raid took place as the driver stopped off to buy a newspaper at a newsagent's in Long Lane, Chadderton, Oldham, Greater Manchester.
Police confirmed the offender threatened the delivery man waiting in the van and forced him to lower his head before assaulting him.
"I find it extraordinary that the theft of so many passports was even possible"
"Whilst the driver was in the shop, both the doors to the van were opened and the passenger in the van had his head forced down into the dashboard and got told to keep his head down.
"The van was then driven a very short distance ... onto a bit of an unmade street," Detective Chief Inspector Bill McGreavy said.
"The passenger has kept his head down and can then hear noises at the back of the van. And after a couple of minutes, when he raises his head, he can see that there's nobody there."
A spokeswoman for 3M Security Printing & Systems confirmed that no personal details were in the blank documents.
The passport service said the stolen documents could not be used by thieves because of their hi-tech embedded chip security features.
But fraud experts say they can still be used as a form of identification and even for travel in countries where the chip technology is not used.
"That is because they can be used by putting in biographical information of your own, not necessarily getting the chip information right, and then you can use them to open up bank accounts or actually get employment," Tom Craig, an ex-Scotland Yard fraud officer told BBC Radio 4's Today programme.
"The thing is the money, the end user, the end criminal - the guy who is going to get caught - is paying out a lot of money. And you've got desperate people all over the world trying to get British passports," Mr Craig added.
Keith Vaz, a Labour MP and chairman of the influential Commons Home Affairs Committee, wants an inquiry into UK passport security.
"I find it extraordinary that the theft of so many passports was even possible," Mr Vaz said.
"This government has put the eradication of illegal immigration at the top of the political agenda. It is therefore completely unacceptable that such sensitive documents are transported in a way that puts them at risk of theft."
The theft is the latest in a series of security breaches but Labour's deputy leader, Harriet Harman, has defended government policies.
"I think that this is a robbery - a serious crime - and it will be being investigated. But I don't think that it necessarily shows a sloppy attitude. I think it's a crime which is a serious one and will be looked into and we hope obviously - that the police will be able to apprehend the offenders."
The Conservatives regard the theft as another example of lax government security.
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