More than 115,000 passengers were caught up in "widespread confusion, frustration, disruption, discomfort and anxiety" in the Christmas rail engineering overruns, a report by rail regulators said.
Network Rail's (NR) late-finishing work, affecting King's Cross, Paddington and Finsbury Park stations in London, led to "unacceptable" service levels, the report by the Office Of Rail Regulation (ORR) said.
"Passengers were really let down," said ORR's railway markets and economics director Joanna Whittington.
The ORR report into the disruption, affecting services on Saturday December 27 and Sunday December 28, follows NR's own critical report last month which acknowledged the company's failings. Words: PA
The overruns led to King's Cross being closed all day on December 27 and Paddington being closed for part of the day. With trains unable to use King's Cross there were chaotic scenes at Finsbury Park where the crush was so great at one stage that passengers were unable to disembark.
In today's report the ORR said:
:: The planning of the King's Cross work did not take proper account of the impact of failing to hand back a working line on December 27;
:: A contingency plan developed on the evening of Boxing Day was ineffective and resulted in Finsbury Park becoming so overcrowded it had to be closed for a time;
:: Reporting of the progress of the works at Paddington was inaccurate so that the operational contingency plan was not enacted until many passengers had already been severely delayed.
The ORR made a number of recommendations to ensure "that in future passengers do not suffer the same experience".
The report said: "Passengers who travel between Christmas and the new year include families with young children, the elderly and vulnerable often travelling with lots of luggage in an unfamiliar environment.
"Over the two days we estimate that more than 115,000 passengers were affected in some way by this disruption. "
The report went on: "There were impressive examples of good service from individual members of staff but the overall service passengers received was not acceptable.
"It led to widespread confusion, frustration, disruption, discomfort and anxiety."
Ms Whittington said: "NR has rightly acknowledged it didn't do enough for passengers affected by overrunning engineering works at King's Cross and Paddington this Christmas.
"While the company generally has a good record for delivering engineering work on time, in this instance passengers were really let down."
She went on: "Our investigation found that contingency planning did not fully consider the impact of potential overruns on passengers and that this needs to change.
"In future, plans will address the impact on passengers as well as engineering and train operation issues. NR cannot achieve this alone, train operators will also need to play their part."
Ms Whittington added: "NR has committed to ensuring all work undertaken this Easter has a contingency plan which is fit for purpose, and to implement all recommendations in time for engineering works planned over Christmas 2015.
"ORR will audit its progress, to ensure that passengers' needs and safety are central to NR's and train operator contingency arrangements."
NR chief executive Mark Carne said: "Putting passengers at the heart of our planning is absolutely the right thing to do and we recognise that we let people down during the disruption experienced on December 27 and 28.
"Over Christmas, we undertook the biggest programme of engineering and investment work ever, upgrading train lines across the country. The overwhelming majority of this work went to plan but in the case of King's Cross and Paddington we accept we got things wrong.
"Our own report has caused us to look again at the work we have planned this year and the contingency plans we have in place and the ORR's report underlines the necessity of this work."
House of Commons Transport Committee chairman Louise Ellman said: "NR must implement the ORR's recommendations in full so that passengers do not face avoidable disruption and chaos.
"The scenes at Christmas were unacceptable. Bad planning, the failure of contingency arrangements and breakdown in communication resulted in thousands of people waiting for hours in the cold outside Finsbury Park station, or stuck on delayed trains."
She went on: "The rail sector must work together to ensure that NR's planning and communication improves. NR's investment programme for 2014/19 means that more major and complex engineering work will need to take place. NR must learn from their errors to regain the confidence of the public."
Shadow transport secretary Michael Dugher said: "Over 100,000 passengers were affected by the rail chaos over the Christmas period. We now know that David Cameron's Government was warned and failed to act.
"While it is clear that NR needs to learn lessons, out of touch ministers must learn from their failure too."
He went on: "It is also increasingly evident that there needs to be big reforms to the running of our railway so that we have a rail service that is fit for purpose.
"The ORR is right to say that the rail industry must place passengers at the heart of enhanced contingency planning. Labour has argued consistently that passengers must be given a proper voice in the running of the railway, so that they are at the centre of the way in which the industry is run."
Michael Roberts, director general of the Rail Delivery Group which represents operators and NR, said: "Major projects like those completed over Christmas are crucial to improving the railway.
"But many passengers did not get the service they deserved in the days following Christmas and for that we are very sorry. NR and operators are working closely together to avoid making the same mistakes again."
He added: "Passengers are our primary focus and it is vital that the industry provides them with accurate and timely information, particularly during disruption.
"In the coming months train operators will be publishing individual plans to improve customer communications."
Manuel Cortes, leader of the TSSA rail union, said: "This report reveals that the post-Christmas chaos was such a shambles because NR tried to substitute a small suburban station, Finsbury Park, for mainline King's Cross which has 12 platforms.
"Any schoolboy knows you cannot get a quart into a pint pot. For some reason, no one at the top of NR seemed to know that basic fact.
"When they tried to ignore this very basic sum, tens of thousands of passengers had their holidays ruined."
David Sidebottom, director at rail customer watchdog Passenger Focus, said: "Thousands of passengers were left angry and frustrated when trying to travel at Christmas.
"We heard of passengers standing for hours on trains, locked outside stations and left to find out what to do by themselves."
He went on: "This cannot happen again. We welcome this report from the regulator, which echoes our call for industry to put passengers' experience right at the heart of planning, especially for plans to cope and compensate when things go wrong."
Transport Secretary Patrick McLoughlin said: "Passengers should be at the heart of everything the rail industry does. The industry must learn lessons to avoid any repeat of the completely unacceptable disruption we saw in December.
"NR and the operators are delivering essential improvements that we are investing in as part of our long-term economic plan. However, it is vital they ensure these works are properly planned and any impacts to services are communicated to passengers."