New trains being introduced on the East Coast Main Line are not fully compatible with existing infrastructure, it has been revealed.
Azuma trains have been found in tests to not work properly with signalling equipment.
It is understood that the trains are interfering with signalling - which acts as the railway's traffic light system - resulting in it being put out of use or giving false readings.
The Class 800 Intercity Express (IEP) trains are due to begin operation by London North Eastern Railway (LNER) on the East Coast route - which connects the North East with London, York and Edinburgh - from December.
The operator insisted it is still planning on meeting this date, but Transport Secretary Chris Grayling said the trains would "start arriving next year".
A spokeswoman for Network Rail, the Government-owned company responsible for Britain's rail infrastructure, said "electromagnetic emissions from the train" are interfering with signalling.
She went on: "It is Hitachi's responsibility to demonstrate that the IEP trains can run safely on the East Coast Main Line, accordingly we believe that this issue should therefore be fixed on the train.
"However, in the interests of finding a practical solution to ensure passengers benefit from the new trains, Network Rail is looking at mitigation to line-side infrastructure - alongside further modifications to the trains - which will be subject to further testing by Hitachi to demonstrate compatibility."
A Hitachi spokesman said there are a number of signalling systems on the route which are 30 years old and require modifying to operate with modern electric trains.
"Network Rail is planning to carry out this modification work before the Azuma trains enter into passenger service," he said. "This is the same issue encountered 15 years ago when the Pendolino was introduced on the West Coast Main Line."
Class 800 IEP trains were launched in the UK on the Great Western Main Line in October 2017. The first scheduled journey suffered a leak from an air conditioning unit and further issues resulted in some trains being temporarily withdrawn from service.
Asked about the issue with the Azumas, Mr Grayling told BBC Inside Out: "We've had teething problems here, just as we've had teething problems on the Great Western Line.
"We've started to move now towards the greater integration of track and trains. The new franchises involve much closer working with Network Rail - the track operator - and the train companies, but we're going to get brand new trains working - they start arriving next year."
Publicly owned LNER began operations in June after Virgin Trains East Coast - a joint venture between Stagecoach (90%) and Virgin (10%) - saw its contract for the East Coast ended prematurely after they failed to achieve revenue targets.