Your contract of employment should state the amount of notice you are required to give. The law states that that period should be a minimum of a week's notice after you have completed a month's service, but your contract may require you to give a longer period. If without justification you leave and don’t give adequate notice, your employer may seek to retaliate. It cannot make deductions from your final pay (although some employers might try this) but could (very unlikely in most circumstances) pursue you in the county court for breach of contract (eg the cost of hiring a replacement for you during your notice period). Also your employer could legitimately mention this in any reference you seek from it.
