Business chiefs have warned of the risk of widespread disruption at Channel ports after Brexit because new IT systems set up to handle the transit of more than 17,000 goods a day will not work.
Preparations for checks on goods entering and leaving the UK from Kent are likely to cause widespread chaos with the new 'untested, incomplete' system.
With 6.3 million movements of goods estimated under transit arrangements every year - just over 17,000 a day - the new network is said to take 45 minutes longer than currently for a single customs declaration.
The Customs Declarations Service (CDS) replaces the old Customs Handling of Import and Export Freight (CHIEF) system.
Des Hiscock, director general of The UK Association for International Trade, warned the consequences could be catastrophic and added the new system 'does not make sense'.
His warning came after the Government itself warned in September there could be Operation Stack queues of up to 7,000 trucks in Kent and two-day trade delays in a worst-case scenario letter sent by Michael Gove to the freight industry.
In the scenario, between 30-50 per cent of trucks crossing the channel would not be ready for the new regulations which will come into force on January 1, 2021.
Photos taken in October showed a vast 27-acre Kent field being turned into a lorry park for up to 2,000 trucks in case of mass hold-ups at Dover.
And just last week, the National Audit Office said that although the Government had made progress updating customs systems and other infrastructure it still expected 'widespread disruption' after the transition period ends.
Mr Hiscock said: 'We need to agree a co-ordinated mitigation plan with HMRC and this industry if we are to achieve any success.
'There are two challenges that need to be addressed.
'The first is obviously the UK's unnecessary insistence on pursuing this untested and incomplete system; it is not going to be ready; it is unreliable; its functionality has been significantly reduced.
'It takes 45 times longer to complete a CDS declaration. It just does not make sense.'
He added that legitimate businesses would be operating on an unfair import playing field as goods entered the UK unchecked for the first six months after January 1.
'This is definitely going to be abused... but more importantly, it is totally unnecessary.'
The warning was made during a House of Lords select committee hearing examining contingency plans that could be implemented after Brexit in January.
Peers were told the new IT system was not ready and was unlikely to be ahead of January.
The National Audit Office also said the Covid-19 pandemic has 'exacerbated delays in government preparations and significant risks remain'.
The report said the government was aware of the prospect of delays and was putting in place plans to monitor issues but there was still a 'risk that widespread disruption would ensue'.
Commenting on the report, Public Accounts Committee chair Meg Hillier said it was clear that whatever happens 'there will be disruption at the border in January'.
She said: 'The government simply hasn't given businesses enough time to prepare.'
Last night, the House of Lords rejected moves which would have given ministers powers to breach the Withdrawal Agreement brokered with Brussels last year.
Critics warned the clauses, in the Government's internal market bill, flout international law but ministers said they were necessary in case the EU abuses the terms of the agreement to prevent food exports from the mainland UK to Northern Ireland.
Conservative former leader Lord Howard and former Chancellor and Home Secretary Ken Clarke were among senior Tories to rebel against the Government.
The House of Lords voted 433 to 165, majority 268, to remove clause 42 - one of the disputed sections - and clause 43 was removed without a vote.
They then voted 407 votes to 148, majority 259 to remove clause 44, relating to the Northern Ireland Protocol.
All the other controversial clauses were removed without votes.
The Government promised to reinstate the clauses when the legislation returns to the House of Commons next months.
This is despite fears it could create tensions with US President-elect Joe Biden.
Mr Biden previously raised concerns about the Good Friday Agreement.