Passengers faced with no Edinburgh-London King's Cross trains over the English August bank holiday weekend are now warned services on an alternative route will be full.
They are likely to have to change trains in Glasgow and complete journeys to and from Edinburgh by ScotRail.
The news comes days after The Scotsman revealed Edinburgh festival chiefs said they faced “total disaster” over the closure of King’s Cross for engineering work over their events’ final weekend.
LNER has already urged passengers not to travel on Saturday 24 or Sunday 25 August, with only a limited service south of York to and from Peterborough on the east coast main line.
Services will also be “significantly reduced” the following day.
However, Virgin Trains has now warned people not to use its trains between London Euston and Edinburgh either.
A spokesperson said: “With a number of events taking place along our route during the August Bank Holiday weekend and planned works being carried out by Network Rail, our services to and from Scotland are expected to be extremely busy.
“We strongly recommend customers who wish to travel to the Edinburgh Festival on our west coast route make use of journey opportunities via Glasgow, make a seat reservation and plan their journey in advance.”
Virgin operates Euston-Edinburgh trains via Birmingham, but it said passengers may also be unable to get seats to Edinburgh by changing from London-Glasgow trains at Crewe or Preston.
Travelling via Glasgow will add more than an hour to journeys and could also be more expensive.
The King’s Cross closure is for the renewal and streamlining of tracks and overhead power lines on the station approach, along with the reopening of a disused tunnel.
An LNER spokesperson said: “These Network Rail engineering works will cause significant disruption across the LNER route
“As a result, LNER is advising customers not to travel to or from London on these days.”
In a leaked joint letter, International Festival, Fringe, Tattoo and book festival chiefs wrote that it would have a “massively detrimental effect.
“It cannot be acceptable for Network Rail to set a timescale that will be totally disastrous for the Edinburgh festivals.”
David Sidebottom, director of passenger watchdog Transport Focus said: “For passengers wanting to travel over the bank holiday to the Edinburgh Fringe, engineering works could be a major inconvenience, especially if it means having to make longer, unfamiliar journeys while London King’s Cross is closed.”
He said people understood such work was necessary, but it was vital passengers were given “accurate, timely and useful information to help them make informed decisions” and there were extra staff on hand at stations.