A man is being questioned in connection with the theft of 3,000 blank passports and visas worth about £2.5m from a security van in Oldham.
It follows an alleged attack on a security van after it stopped at a newsagents shop on 28 July.
The man arrested is understood to be the delivery man who accompanied the driver on his journey.
The Foreign Office has said there was a serious breach of security over the loss of the documents.
A statement from Greater Manchester Police (GMP) said: "A 48-year-old man from Oldham was arrested on suspicion of conspiracy to commit robbery and is currently in police custody."
Earlier this week officers described how the delivery man was attacked by an unknown offender who drove the vehicle a short distance before escaping.
Passports 'unusable'
Police said the white Citroen van was carrying 24 brown cardboard boxes, although not all of the boxes in the van were taken.
The stolen passports were destined for embassies abroad and were being taken from 3M Security Printing & Systems in Chadderton to RAF Northolt.
The delivery, in the non-armoured van, was being carried out by a private company on behalf of the Foreign and Commonwealth Office (FCO).
Police, who estimated the black market value of the passports at £2.5m, said they were "very secure" because they contained a microchip which could be encrypted.
And the Identity and Passport Service (IPS) insisted the embedded chip security features rendered the passports "unusable".
Greater Manchester Police are continuing to appeal for witnesses to the robbery, which happened just before 0640 BST on Monday.
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