UK customs officials are battling to resolve an outage in a key post-Brexit IT system, with drivers complaining that the malfunction is adding to long delays for freight traffic trying to cross the Channel.

A surge in Easter tourist traffic and reduced sailings from Dover, where several P&O ferries are still out of commission after the mass sacking of crew members, are combining with delays at customs to create havoc on the roads into the port, with parts of the coastbound M20 motorway in Kent turned into a temporary lorry park as HGVs wait to reach France.

Haulage industry insiders have put part of the blame for the gridlock on the week-long outage of one of the government’s new trade portals, the goods vehicle movement service (GVMS). It first experienced problems on 30 March and has not been available for use by traders and hauliers since the following day. Drivers are being warned it may not be up and running until Monday.

GVMS, which was developed by and is the responsibility of HM Revenue and Customs (HMRC), has been suffering a systems outage since it was temporarily taken offline during an update to another government customs system, handling import and export freight, known as Chief.

Ciaran Donovan, a driver based in the UK who regularly moves goods back and forth between Britain and the continent, said he was unable to use the GVMS portal to get the goods movement reference (GMR) number – a barcode – required to leave the UK last Wednesday. “I had to cancel a job out to Paris on Thursday, which was annoying,” he said.

GVMS users trying to access the service have been notified on at least two occasions over the past week that the system would soon be operational again. However, those dates have come and gone and the government has said GVMS will not be up and running again until midday on Monday.

Traders trying to access GVMS are notified that HMRC is “still investigating ongoing issues” with the service.

In the meantime, HMRC has implemented contingency measures, allowing hauliers to show other evidence that a customs declaration has been made to take goods out of or bring them into the UK, including a transit accompanying document or a movement reference number.

Despite these interim measures, truckers say the requirement to provide different customs information is likely to add to processing time at the border. The GMR barcode is scannable and thought to save time.

An HMRC spokesperson said: “Our IT systems are stable: we have put in place contingency processes to ensure businesses can keep goods and freight moving while we return to full service.”

They apologised for any inconvenience and said they would work closely with customers to avoid disruption, adding that HMRC took “system resilience very seriously”.

Donovan said current disruption meant he was working out whether to accept future courier jobs. “I would rather not work than sit there for 20 hours trying to leave the country,” he said.

Duncan Buchanan, the policy director at the Road Haulage Association, described the week-long outage to GVMS as “really unhelpful” at a time of an “accumulation” of other problems.

The vessels usually operated by P&O Ferries are still out of action after the company’s mass sacking of 800 workers last month, representing a third of Dover’s usual ferry capacity. Bad weather has also delayed some sailings in recent days, just as tourist traffic booms before the Easter holidays, and the traffic backlog was further increased by the breakdown of a Eurotunnel train on Monday.

Drivers faced another day of traffic chaos on Thursday as lorries queued for hours to reach Dover port and the Eurotunnel terminal, and Kent residents faced disruption and diversions.

A 23-mile stretch of the M20 from junction eight near Maidstone to junction 11 at Westenhanger was closed on Thursday to all non-freight traffic as part of the Operation Brock traffic management system. It was estimated that more than 2,000 lorries were stuck in the queue.

“One member reported that they joined the queue at 8pm last night and this morning just before 9am they were still 30 miles from Dover,” said Buchanan, adding that lorry drivers were being forced to wait in their cabs without access to toilets or places to buy food and drink.

“We are getting lots of feedback from drivers who are complaining about the absence of facilities. They are basically being advised to take their statutory rest before going to Kent,” he said.

Days of traffic chaos have also affected Kent residents and businesses, said Toby Howe, the tactical lead at the Kent Resilience Forum, a partnership of agencies and organisations. “We basically have more lorries coming into the county than can get out the other end,” Howe said. “This issue isn’t going to go away, and it is always Kent and its residents who have to pay the price if there are any problems on the Channel crossings.”

Trevor Bartlett, the leader of Dover district council, said in an open letter that the port would remain under severe pressure up to Easter, and he would not hesitate to “declare a major incident” if Dover was again snarled up by port traffic, because of local fears that this could hamper the emergency services.