FirstGroup announced it had ordered five Hitachi AT300 trains to take on LNER from autumn 2021.
It plans five or six services a day on the east coast main line between the cities, only stopping at Morpeth, Newcastle and Stevenage.
First said fares would average less than £25 with end-to-end journey times of around four hours.
The operator - led by former ScotRail managing director Steve Montgomery - said it will be able to offer lower fares by running fewer and shorter but fuller trains than LNER.
LNER is itself due to start operating new Hitachi Azuma trains between Edinburgh and London from late summer, after initial runs between London and Leeds from 15 May.
The five five-carriage FirstGroup trains are part of £100 million financed by Beacon Rail, from whom they will be leased.
They are similar to the trains that sister firm TransPennine Express is due to introduce in the north of England, including on routes to Edinburgh and Glasgow, at the end of the year.
The operator said it secured rights from the Office of Road and Rail to run its new "open access" service between the two capitals similar to its Hull Trains service to London.
BACKGROUND: FirstGroup to challenge Virgin with £25 Scotland-London fareMr Montgomery, managing director of First Rail, said: “There’s a real gap in the market for truly affordable rail travel between the two capitals.
"Our plans show we are serious about competing with low-cost airlines and opening up rail travel on this key route to thousands of new passengers.
“The new trains we are introducing will provide the highest level of comfort and service, whilst being great for the environment and air quality.”
Karen Boswell, managing director of Hitachi Rail Europe, said: "Our Japanese-bullet train inspired fleet will play a vital role in supporting FirstGroup’s exciting new service.
“These trains have been designed to increase the number of seats available whilst harnessing the latest in comfort design to boost passenger experiences."