It hasn’t yet been worked out whether the upcoming Crash Course campaign will work on Left 4 Dead 2, meaning the predecessor vs sequel matches might not happen.

"We're still struggling with how interoperability is going to work, right now," Chet Faliszek, the game’s writer told Edge. "When we talk about L4D, the story stuff - there's actually a lot of character dialogue that's based in each map".

There’s a worry that the characters in the second game wouldn’t, in any consistent reality, have any reason to be in the same area as those in the first. This includes the area Crash Course is based in, which would be where these games would have to be set in.

"We have a world sketched out and we have an understanding of where everything is and what's going on, you know, so story-wise it wouldn't make much sense," Faliszek added. "But equally there's a whole group of the fans who don't care."

Valve thought about bringing the first game’s characters to the second. "That's one of the routes we've talked about going, and maybe adding the new creatures, or whatever". It seems unlikely that this will be finished in time with Left 4 Dead 2 out in November and Crash Course out this month.

The focus on Versus was a response to feedback and statistics, mainly the fact that the games were lasting too long. The DLC should keep Versus matches to 20 or 30 minutes long, which is hoped to appease fans.

Crash Course is "a two-map campaign with a finale", and is set to between the No Mercy and Death Toll campaigns. There are extra Survival maps and the whole thing can be played through, as you’d hope, co-operatively.

Crash Course will be released for free on PC, Xbox 360 owners will have to pay 560 MSP.