The scheme was set up following the end of the Brexit transition period to give businesses importing into the UK up to 175 days to complete their customs declarations, with the first of those due on June 25, reports London's Air Cargo News.
But to submit delayed declarations, companies first need to be Customs Freight Simplified Procedures (CFSP) authorised.
The British International Freight Association (BIFA) has warned that some importers that have not taken out a CFSP authorisation are now trying to find customs agents to undertake the work required to complete the process.
However, agents could potentially be liable for duties on any declarations they take on board.
'We are reminding members that if they take on this work, it is their CFSP authorisation being used and they may have to pay any duties and VAT on behalf of the importer,' said BIFA director general Robert Keen.
'So, it is essential that they ensure that their customers' paperwork is in order and completely accurate, and if it is not, our advice is to not accept the responsibility,' he said.
'Whilst the government has extended the scheme to defer declarations to December 31, 2021, on a rolling basis, it is important that any BIFA members undertaking this work on an importer's behalf remain vigilant and ensure that they have all the information required to make an accurate supplementary declaration and that they don't miss the first and subsequent deadlines.
'Any mistakes could be costly as, despite the ability to use direct representation, the authorised agent is still considered to be the owner of the procedure and responsible for a timely submission of correct declaration