Employer change pay period without notice?

My employer has told me that he is changing our pay period. We get paid last working day of each month but now decided he wants to change the cut off day to 23rd of each month. So this month means I am only getting paid 3 weeks worth of pay. He states he is doing nothing wrong as contract says you will get paid last working day of month in arrears. But surely it s not all arrears as cut off point changed from last day to a week before? I have a mortgage ect and can t afford to live on 3 weeks pay. Please tell me where I stand?

Read more: Employer change pay period without notice?