Eurostar passengers have been forced to wait in a queue which snaked along roads outside London's St Pancras station.
Author Svenja O'Donnell, who was among the passengers in the queue, said she waited in the line for nearly three hours for a train to Paris on Tuesday.
She said: "The queue is through the whole station and down two blocks. We have been given very little information."
She advised other passenger to "stay home" to avoid the "absolute chaos".
The pandemonium continued on Wednesday, with one traveller saying the queue outside the building stretched back all the way to Regent's Canal.
Those travelling on the service this morning were facing 90 minute waits for check in, with one passenger being advised when to join the queue by staff.
He wrote on Twitter: "Nice chap advised me to join an 1 and 1/2 hours before departure. Luckily I'm not due to join it for a while yet..."
When asked if it was normally like this, the regular Eurostar traveller replied: "It's normally very quick, rock up 30 mins before departure.
"Combination of Covid checks, not being re-staffed to full strength yet, and people desperate to escape the Jubilee."
On Tuesday at least one train from London to Paris departed with none of its onboard toilets working.
Another Eurostar customer described the situation at St Pancras as a "total shambles", adding: "Thousands of people, queues snaking around the station, out into the street and back in again. Shocking mismanagement."
One traveller complained about "waiting outside in the rain" and there being "no clear signs and explanation".
On Tuesday Eurostar said in a statement: "Unfortunately we had a technical fault on a train earlier this morning which meant we had to take it out of service.
"As a consequence, there are delays of 30-60 minutes on departure in London St Pancras.
"Queues are moving and our staff are working hard to look after passengers and ensure their check-in is as smooth as possible."
The Paris Airport authority, which manages Charles de Gaulle and Orly airports, and the Eurostar rail service both warned on Wednesday of major disruption to travellers, adding to pressure on holidaymakers already facing delays across Europe.
The Paris Airport body sent a message on its Twitter account saying that software problems were impacting border controls checks, and this in turn would lead to delays.
Eurostar also issued a similar warning.
"Our stations are very busy today. Passport and security checks are taking longer than usual due to issues with French authority control systems," it said on its Twitter account.
Airports across Europe have struggled to cope with a post-pandemic rebound in demand, but British travel hubs are facing a particularly difficult week as a school half-term holiday combines with a long public holiday weekend to mark Queen Elizabeth's 70 years on the throne.
