Red light for trains.
The claim, in a report by Network Rail, is a blow to plans by Grand Central Railway to run new services between London and Sunderland, via Teesside.
All but one of GNER's planned new six daily services to Leeds have also been given a red light until the track system is upgraded.
The major problem limiting any new trains is the lack of platforms available at King's Cross Station.
A Network Rail spokesman said: "The East Coast Main Line is one of the busiest rail routes in Europe.
"Without further infrastructure enhancements, it will be very difficult to deliver the additional services."
GNER spokesman John Gelson said it would be harder for them to meet their contract agreements for the line.