A single operator will run the current InterCity West Coast main line along with new HS2 high-speed services, said the government.
The Department for Transport said it would create a new franchise called the West Coast Partnership, which will take over the West Coast service from 2019 and run high-speed trains from 2026.
Construction of the £55.7bn ($69.6bn) HS2 line is scheduled to begin next year, and will eventually link London, Birmingham, the West Midlands, Leeds, Sheffield and Manchester.
The new franchise will run for the first three to five years of HS2's operation.
The West Coast service is currently run by a joint venture between Virgin Trains and Stagecoach. Tenders for the new franchise will open in October or November next year, the Department for Transport said.
Transport Minister Andrew Jones said: "HS2 will be the backbone of Britain's railways, creating more seats for passengers on the West Coast and increasing capacity on the rest of the network.
"By combining the franchise we are ensuring we get the right people on board at an early stage to design and manage the running of both services in the transition stage. The new franchise will attract highly experienced companies, who have the right experience, which ultimately means a better deal for passengers – both now and in the future."
Passenger fare protection 'a priority'
The government says the new rail line will lead to a more competitive regional economy.
It also adds that phase one of the project, between London and the West Midlands, will almost triple the number of seats at rush hour from 11,000 to around 30,000.
Construction of HS2 phase one is due to begin next year with the first trains due to run between London and Birmingham nine years later. Phase two of HS2, between the West Midlands and Manchester, is set to begin operating trains in 2033.
Anthony Smith, chief executive of the independent watchdog Transport Focus, said because a single firm would run both services passenger protection on fares "will have to be a priority".
Smith added: "Passengers now and in the future will be pleased to see a co-ordinated approach to delivering services running up the west coast as the new line opens up more capacity.
"With a franchise of this size, it is even more critical that passengers are kept at the very heart of it, and that their satisfaction is built into delivery targets. Less competition could too easily lead to premium pricing so passenger protection will have to be a priority."