Failure to pay wages for work done counts, in law, as an unauthorised deduction from wages. If the matter cannot be resolved between you and your employer, you have three months to bring an employment tribunal claim for your unpaid wages.
The first step is to submit an Acas Early Conciliation Notification Form. There is more information on the Acas website site. It is possible that the Acas conciliation officer will be able to persuade your employer to pay what is owed without the need for a tribunal claim.
Speak to your union rep as soon as possible. If you are not a union member, browse our Union Finder tool to find out which union best suits your needs and to discover more about organising to secure union recognition at your workplace. You should also contact your local Citizens' Advice.
Your employer may be insolvent. There is advice here about how to claim payments from the government Insolvency Service if your employer is insolvent.