Regulation (EC) No. 1889/2005 requires all individuals who bring cash into or out of the EU to declare these amounts to the customs authorities in the Member State which they first enter or leave. The data is compiled centrally and used by the EU Member States to identify risks such as money laundering. Following the UK's exit from the EU, the EU Regulation will no longer apply to the UK as a non-member state. In order to ensure that the UK is able to continue to collect data about the movement of cash in excess of £10,000, amendments need to be made to retained EU law.
Also see EU Exit SI tracker