Virgin Trains West Coast is offering 60,000 first class seats from £12.
The sale prices are available on routes from London to Birmingham, the north-west, north Wales and Scotland.
The promotion is scheduled to end on December 3 and the discounted fares are valid for travel between December 12 and January 23.
First class includes complimentary lounge access, free on-board food and drink and wifi, as well as larger seats and more legroom than in Standard carriages.
Tickets are bookable only on Virgin Trains' website, where passengers can also earn Nectar points.
Advance tickets are non-refundable, but are changeable up to the time of departure for a fee of £10 plus any difference in fare. Tickets issued as e-tickets are non-changeable, however.
For more information, click here.