A Network Rail report has identified a bottleneck north of the planned line, due to be constructed between London and Birmingham by 2026, which will constrain how many through services can travel to the north of England and Scotland.
Modelling for the UK Government's High Speed Two (HS2) rail network assumed phase one would provide up to four trains per hour from London to Birmingham, six to the north of England and one to Scotland, in addition to existing passenger services.
However, the predicted growth of rail freight on the West Coast Main Line (WCML), Europe's busiest railway, will act as a significant brake on the potential for new cross-border rail services, Network Rail has concluded.
Its report, Future Priorities for the West Coast Main Line: Released capacity from a potential high-speed line, found either the extra freight trains could be accommodated or the additional passenger trains, but not both.
Proposed solutions include limiting the number of long-distance freight services or new passenger trains. A third option, which Network Rail said had been discarded, was to divert cross-border trains via Birmingham, adding to the journey time.
Backers of the new project, such as the UK Government, say the benefits will be felt not only by those using the route but other rail users, who will see more services as space is freed up.
However, the study suggests the situation will be complex as the new line will create bottlenecks as well as extra services.
The report stated: "The route section just north of where HS2 phase one joins the WCML [Lichfield] will be the foremost constraint on additional through services on the WCML prior to implementation of the second stage of Government's proposals for high-speed rail. The number of additional through services will be limited by rail freight growth, and vice versa."
The Scottish Government is in talks with Westminster over how to extend the route to Scotland, with a timescale to be agreed before the Holyrood elections.
Christian Wolmar, a prominent railway commentator, said: "The notion the high-speed rail line is a panacea to all of rail's problems is false. This shows there may be unintended consequences."
The director of rail watchdog Passenger Focus, David Side-bottom, said: "A balance must be struck between the needs of passenger and freight services. In the longer term, more capacity must be delivered."
Meanwhile, Chris MacRae, rail freight manager for the Freight Trade Association, said his group was concerned by the capacity of the West Coast Main Line to take on more freight trains and high-speed rail services.
He said: "North of Preston it becomes largely two-track and you've got to fit higher-speed passenger trains with lower- speed freight trains. One issue is when the high-speed line goes to Birmingham there will be more passenger trains going north because of pressure to increase Scotland's connectivity so it's not marginalised."
A Network Rail spokesman said: "We are examining options for services through Lichfield and the changes that may be required once high-speed trains are in operation. This is still at an early stage, but we are mindful of the need to effectively accommodate freight and passenger traffic."