Be prepared to download a 10GB day one patch for the retail version of Sniper Elite 3 for Xbox One.

Rebellion's first-person shooter, out this week, carries a mandatory install of just shy of 20GB.

That mandatory install was going to be be 21GB, but 14GB of it will be replaced by the day one patch.

Rebellion told Eurogamer the patch makes various improvements to the game, and actually brings the whole game install size down to 19GB rather than 21GB.

But why have such a huge day one patch in the first place? It seems it has to do with the way the Xbox One currently handles game updates.

Here's Rebellion's official explanation:

"The main part of the update is to sync data between the game's retail discs and the Xbox Live digital download edition of the game. In the several weeks between sending the code off to manufacture and release, rather than sit around or take a break, we've continued to work on the game, making improvements and optimisations.

"We've also begun work on multiplayer DLC and a small part of this update is to introduce a compatibility pack. This is standard for any game that adds DLC - it's just to make sure everyone can have the same multiplayer experience whether they own DLC or not."

Sniper Elite 3's 10GB day one patch is one of the largest we've ever seen, and was discovered by Eurogamer's YouTube Editor Ian Higton today when he tried to play the Xbox One version of the game.

And for those with a modest broadband internet connection, it'll take a while to download and thus play the game.

Rebellion acknowledged this.

"While we understand gamers will be frustrated at the wait, the good news is that the update will actually reduce the size of the game's total install footprint on their hard drive, and reduce the size of future patches significantly.

"It's part of this ongoing optimisation process that's enabled Sniper Elite 3 to perform well at a native 1080p on both Xbox One and PS4. We appreciate everyone's patience and we firmly believe Sniper Elite 3 will be worth the wait!"

Day one patches will apply to the other versions - on PlayStation 4, PlayStation 3 and Xbox 360, but they will be much smaller, Rebellion said.

The developer also pointed out that the 10GB patch only applies to the first batch of retail discs. If you buy Sniper Elite 3 from the Xbox Marketplace directly, or get a second wave disc, as it's called, you do not need to download the update.