East Coast: "Overrunning engineering works by Network Rail north of London King's Cross have resulted in severe delays to our services. We are very sorry about this and advise customers to travel on another day if at all possible. Tickets for today are valid on Sunday 28 and Monday 29 December."
Grand Central trains "Tickets for travel on Grand Central trains on Saturday 27 and Sunday 28 can be used on Sunday 28 and Monday 29 December, and customers are encouraged to delay their journeys if possible."
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The family of a man who has been missing since last weekend has been posters in the area where he was last seen.
Colin Pine, 55, of Ennerdale in Albany, Washington, failed to return home after going out to watch the Tyne-Wear derby on December 21.
He was reported missing in the early hours of December 22.
He watched the match at The Swan in Felling, where he was until Sunday evening.
Finsbury Park station in London has been temporarily closed due to overcrowding, believed to have been partly prompted by the closure of Kings Cross.
Several passengers tweeted pictures of large crowds left waiting outside the station and on the platforms.
Meanwhile, over-running rail works have also closed all lines in and out of Paddington, First Great Western reported.
Over-running rail works have closed all lines in/out of London Paddington. Significant disruptions and delays likely to all such journeys.
David Sidebottom, passenger director at the independent watchdog Passenger Focus, said: "Investment in maintenance and improvement is necessary, and we passengers understand that.
"But overrunning works that disrupt already-limited festive travel are frustrating.
"Our research is clear: passengers want to be kept on the train wherever possible, they want to know before buying a ticket if part of the journey will be by bus, and they want plenty of staff on hand to signpost where to go and what to do.
"We will be looking to see that operators and Network Rail are doing all in their power to alert passengers, to help them make alternative arrangements and to make it easy for them to claim refunds or compensation."
Network Rail confirmed that trains to and from King's Cross would be cancelled tomorrow due to the overrunning engineering works.
The railways operator said that the work was part of a £200 million Christmas investment programme.
It is one of 300 projects being undertaken over the holidays across 2,000 sites up and down the country by some 11,000 railway engineers.
A spokesman said: "What has happened is really regrettable and unfortunate, but it is a small part of a massive amount of engineering investment taking place over Christmas."
Train services in and out of London King's Cross station have been cancelled tomorrow because of overrunning Network Rail engineering works.
Sources said that the station would effectively be closed throughout the day due to delays to works north of the station.
East Coast Trains said customers would need to start or finish their journey at Finsbury Park in north London, with at least one change of train.
A reduced service to and from the station will go ahead as planned on Sunday, with trains leaving up to 20 minutes earlier than normal from King's Cross.
East Coast spokesman, Paul Emberley, said: "Network Rail has apologised to passengers for the inevitable delays to their travel plans on Saturday as a result of the overrunning engineering works.
"East Coast is particularly sorry too for the inconvenience to its customers as a result, on what we know is an already very busy travel day immediately following the Christmas break.
"For customers intending to start or finish their journey at King's Cross, consideration should be given to deferring travel plans to either Sunday or Monday.
"We're working hard over the holiday period to make the necessary adjustments to our timetable as a consequence, and to provide as much information as we can."
National Rail said on its website that services to and from King's Cross would be "significantly disrupted" tomorrow.
It said: "A revised timetable is currently being developed and will be made available as soon as possible. Please check our website regularly for updates."
"Advance warning for Saturday 27th December: Due to overrunning engineering works north of London Kings Cross, we are expecting significant disruption to East Coast services.
"Customers are advised to defer travel where possible.
"A revised timetable is currently being developed and will be made available as soon as possible.
"Please check our websiteregularly for updates."