Your contract of employment should state how much notice you must give. The law states that the 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.
Generally, your employer is not allowed to withhold wages to take account of your failure to work your full notice, unless your contract contains a clear written term that allows them to do this.
If you leave without giving the full notice under your contract, you run the risk of harming any reference given by your employer.
Speak to your union rep.