The report said Eurostar had not prepared properly for the snow-ingestion which caused five trains to break down and had to "improvise" as it had no plan to deal with such an emergency.
Passengers, including expectant mothers and youngsters returning from Disneyland Paris, had to contend with darkness, extreme heat, hunger and a lack of information.
On the stranded "Disney train", with 664 people on board, parents had to strip their children down to underclothes and nappies because of the heat.
Later, having transferred to a Eurotunnel passenger shuttle in the tunnel, the same passengers were confronted with cold and dirty conditions, with "pregnant women and small children having to sit on greasy floors or lean against the side of the carriage".
Toilets on this train overflowed, forcing passengers to designate one carriage as an open toilet area.
Conditions on this shuttle were "appalling" and Eurostar had been "found wanting", said former GNER East Coast Main Line rail boss Christopher Garnett who produced today's report with French transport expert Claude Gressier.
Making 21 recommendations, including train modifications, improved communications and better emergency plans, the two men said: "Passengers must not go through this again."
Some passengers took as long as 17 hours to complete their journeys and Eurostar had to cancel all services on December 19, 20 and 21 before resuming a reduced operation on Tuesday December 22.
Following the publication of the report today, Eurostar said it had modified its trains and more work on them was being done. The company was spending £30 million, including £12 million on a new communication system.
Eurostar chief executive Richard Brown said: "I know we let our passengers down before Christmas and I am determined to put things right."
Transport Secretary Lord Adonis said "too many passengers were left to endure appalling conditions" while rail customer watchdog Passenger Focus said the report confirmed "how badly passengers were let down".
The report said that, in reviewing the causes of the breakdown of the trains, it had become apparent that the standard winter-weather procedures followed by Eurostar were "not suited to the actual weather conditions experienced".
The report added that the problems of snow getting into the trains' works and the build-up of condensation when passing into the warm tunnel from the cold outside "were not fully recognised at the time when the Eurostar power cars were being designed".
The report also told of hours of confusion and further delays and frustrations for passengers through the night and into the morning of December 19 before they were able to get home.
Some failed to fully understand the French-accented announcements, others felt they were being held "captive" on the trains and many complained about lack of food and a lack of information.
The report said Eurostar should have been better prepared for the disruption and reacted earlier.
"The fact remains that Eurostar did not have a plan in place and to improvise, and its provision of information to customers was inadequate", the report said.
Mr Garnett and Mr Gressier said: "This incident caused some passengers distress and others enormous disruption to their holiday plans at a critical time. We believe there are three lessons to be learned.
"First, passengers need to be assured that the Eurostar trains are reliable and so improvements need to be made in this area as a priority.
"Second, if a train breaks down and passengers have to be rescued or evacuated, this must be done with greater speed and consideration, and more comprehensive emergency plans should be put in place.
"Third, in an emergency, passengers need to have prompt information and regular updates. Eurostar must improve the way it communicates with passengers and put in place new systems and practices to achieve that."
Eurostar said: "We fully accept that the handling of the disruption was unacceptable and are very sorry for the inconvenience and discomfort that we caused to our passengers."
It said it had committed to implementing all the recommendations "and a series of actions is already under way to address the issues raised".
Mr Brown said: "Our priority is to win back the confidence of our passengers by taking all the action necessary to prevent this ever happening again."
Lord Adonis echoed Mr Brown's sentiments by saying that Eurostar had to "win back the trust of the public". He added that he had asked Eurostar for an update by the end of March on how it was implementing the report's recommendations.
Shadow transport secretary Theresa Villiers said the incident had caused "untold misery to passengers" and customers deserved to see swift action to "tackle the failures".
Passenger Focus chief executive Anthony Smith said "physical discomfort, emergency evacuations and, above all, poor, if any, information (was) not the way any passenger should be treated".
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