Retailers, shipping and haulage companies have complained of "chaos" at Felixstowe Port in Suffolk, affecting goods in the run-up to Christmas.
One ship due to be unloaded at the port last week was redirected to Rotterdam because of "unacceptable" delays.
The owner, Hutchison Ports, blames pre-Brexit stockpiling and the pandemic.
"It's delaying freight going into shops for consumers to buy,"
Freight manager Matt Hudson warned that "if the chaos continues, increased shipping prices will be passed on to consumers".
Mr Hudson, whose company distributes goods picked up at the port, said containers were being left on the quayside because haulage companies could not book a slot to enter the site.
"It's delaying freight going into shops for consumers to buy," he said. "Shops are struggling anyway at the moment because of the impact from the coronavirus.
"If retail outlets can't sell what is actually en-route at the moment in time for Christmas, potentially they could lose even more money than they have done already this year."
Part of the problem is a shipment of 11,000 containers of PPE ordered by the government that is clogging up the port.
Last week, Taiwanese shipping firm Evergreen directed one of its ships to bypass Felixstowe because of "serious port congestion".
The ship's cargo was unloaded in Rotterdam instead and ferried back to the UK via London's Thamesport.
"We are working with our customers and stakeholders to get through the current congestion"
An Evergreen spokesman said the firm had been told by Felixstowe's owner that a berthing slot - where cargo is unloaded - would not be available for up to 10 days after the ship's scheduled arrival.
"Such a delay is totally unacceptable," he said.
In a statement, Hutchison Ports UK said: "The imbalance in UK trade and Brexit stockpiling exacerbate current operational challenges and we are working with our customers and stakeholders to get through the current congestion.
"Performance at the port remains under pressure due to the Covid pandemic, high levels of import traffic, the large number of empty containers and a large amount of unusually long-stay containers held at the port."
But in an operational note posted on its website, the company said delays at Felixstowe would continue "at least into December and possibly through into the New Year" - potentially causing havoc for firms still waiting on pre-Christmas stock.
In a sign of the company's woes, Hutchison Ports UK recently reinstalled its former boss Chris Lewis as chief executive. He came out of retirement to take up the role for a second time, having last led the firm in 2010.
For months, hauliers have complained about Felixstowe's Vehicle Booking System (VBS) which lorry drivers must use to gain ticketed entry to the port. Freight manager Mr Hudson says in recent weeks, the situation has become worse.
"There are more containers waiting on the port while all trucking companies are logging on trying to get a slot. Sometimes the port cancels some of the VBS [slots] when drivers are actually on port waiting," he said.
"We need to see improvement quickly. It's ruining a lot of people's businesses."
A spokesman for Hutchison Ports UK disputed the claims about the VBS, pointing out that in recent days, empty slots have been available and that improvements to the system are under way.