The current system for processing customs declarations for imports and exports with non-EU countries is changing. The current CHIEF system is to be replaced with the new CDS (Customs Declaration Service) to be phased in from August 2018 into early 2019.
The old system is nearly 25 years old and has become inflexible in a rapidly changing world. Brexit is likely to put demands on the system with an expected increase in the volume of declarations.
You will still be able to process declarations for goods entering or leaving the UK but will need to access the new system via the Government Gateway. There will be a range of improved online services but additional information will be required in some areas.
If you use a customs declaration software package you should check with your software supplier that they will be ready to enable you to make online declarations on CDS.
You can also visit the CDS page on GOV.UK for more information and to register for e-mail updates.