Author Svenja O’Donnell, who was among the passengers in the queue, told the PA news agency she had been waiting in the line for nearly three hours for a train to Paris.
She said: “The queue is through the whole station and down two blocks.
“We have been given very little information.”
Thousands of people, queues snaking around the station, out into the street and back in again. Shocking mismanagement
She advised other passenger to “stay home” to avoid the “absolute chaos”.
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”.
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.”