Despite launching on Steam in other territories, the games are currently unavailable on Valve's digital platform within the UK.
Ubisoft has confirmed that talks between the two companies are ongoing, but has urged fans to pick up the titles from Uplay until the issue is resolved.
"We've been in discussions with Valve about Assassin's Creed 3 and Far Cry 3, but for the time being the games are not available via Steam in the UK," Ubisoft told PCGamesN.
"In the meantime, UK customers wishing to purchase either of these games can do so by visiting the Uplay store, our retail partners or other digital distributors.
"Assassin's Creed 3 and Far Cry 3 are available on Steam in other regions outside the UK," the statement concludes.
It was feared that mainland Europe customers would be unable to purchase the game on launch day, after a shipment was stolen by thieves.
Far Cry 3, meanwhile, launches on the PC and consoles on November 30 in Europe and December 4 in North America.
> Read Digital Spy's Assassin's Creed 3 review
> Read Digital Spy's Far Cry 3 review
Watch a trailer for Far Cry 3 below: