I'm contractually obliged to give my company a month's notice to resign. I have been advised to apply for a job by a friend who says they reckon I'll get it. But they need it filled quite quickly.

What are the pros and cons and legal ramifications of just walking out of a job and into another one? I know it'll probably mean no chance of a reference from this company, but I have an old boss who'd vouch for my best years here, so not too bothered about that. Can they put me in prison or anything?