CDS had resolved the issue

CDS declarations are now able to be submitted. We are still monitoring the situation.

Message from HMRC:

We are aware some customers are experiencing issues with the Customs Declaration Service (CDS). We are investigating and working to resolve the issues as quickly as we can.

If you are using a location using the Goods Vehicle Movement Service (GVMS) you’ll only be able to complete customs movements if you have a finalised Goods Movement Reference. 

When submitting a declaration to CDS imports or exports the response is Internal Server Error. This indicates a problem with CDS itself. We are investigating this as a matter of urgency.