ScotRail will scrap seat reservations on intercity routes next month - prompting fears of 'chaos and disaster' over the festive period.

The policy will come into force on the Glasgow-Edinburgh and Aberdeen-Inverness lines from December 15.

The move, expected to last around six months, is understood to have been prompted by problems with issuing reservations because three different types of trains are being used on the routes.

However a rail union and a transport watchdog hit out at the decision, with concerns of on-board disruption over the holiday season.

A memo to staff from customer experience strategy manager Mark O'Mailley said: "From Sunday, December 15, the way we offer seat reservations will be changing.

“Customers using the Inter-City services from Glasgow and Edinburgh to Aberdeen and Inverness are being advised they will not be able to reserve seats for a period of around six months.

"This means that when a customer purchases a ticket, they will receive a counted place booking for a specific service rather than an actual seat number as they would currently.

"Seat reservations in their current format, will be suspended until the Classic HSTs and older trains have been removed from service and all the new refurbished HSTs are in place.

"We have plans in place to closely manage the capacity of our trains during this time.

"Once the Classic HSTs and older trains are removed from service we will reintroduce consistent, reliable seat reservations on these routes."

The Rail, Maritime and Transport union condemned the move.

Michael Hogg, Scotland regional organiser for the RMT, said: "This policy will lead to conflict on our trains and a recipe for absolute chaos and disaster."

Passenger watchdog Transport Focus urged ScotRail to reconsider the move and said a "free-for-all dash for seats" must be avoided.

Robert Samson, senior stakeholder manager, said: "Although we understand the problems ScotRail are experiencing with the ongoing delay to introducing new trains on inter-city services, to suspend seat reservations on long-distance services in effect means that passengers have no guarantee of a seat over the busy festive period and beyond.

"Groups such as families will be disadvantaged, as will elderly passengers with luggage.

"A free-for-all dash for seats must not be the norm once the boarding process begins.”

ScotRail said the decision was "to avoid any mix-ups with reservations that would not be compatible between the different types of trains in service each day".

Passengers will instead have to find a seat themselves as part of a new temporary "counted places" system.

This is because significant delays with the refurbishment of an acquired fleet of veteran "High Speed Trains" (HSTs) means some of them are still running in their "classic" state, along with some of the trains they will replace.

ScotRail also confirmed the Passenger Assist service, such as for wheelchairs, and bike reservations, would continue to operate normally.