By the 22nd of the month following the tax month if you pay electronically, or the 19th if you still pay by post. Tax months run from the 6th to the 5th, so pay run in the month to 5 May is due by 22 May. Small employers whose average monthly liability is below a threshold can arrange to pay quarterly instead, which is worth having if it applies to you. Late payment attracts penalties that escalate with the number of defaults in the year, plus interest. The most common failure is not inability to pay but forgetting that the money was never yours: PAYE and National Insurance deducted from wages sits in your account looking like working capital, and businesses spend it. Keep it separate.